the active bijection -...
TRANSCRIPT
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A survey on
The Active Bijectionin Graphs, Hyperplane Arrangements,
and Oriented Matroids
Emeric Gioan
CNRS, LIRMM, Universite de Montpellier, France
Joint work withMichel Las Vergnas
A longer online version of the presentation given at:
Workshop on New Directions for the Tutte Polynomial, London, July 2015
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Abstract.
The active bijection maps any directed graph, resp. signed hyperplane arrangement ororiented matroid, on a linearly ordered edge set, resp. ground set, onto one of itsspanning trees, resp. bases. It relates all spanning trees to all orientations of a graph,all bases to all reorientations of an hyperplane arrangement or, more generally, anoriented matroid. It preserves activities: for bases in the sense of Tutte, fororientations in the sense of Las Vergnas, yielding a bijective interpretation of theequality of two expressions of the Tutte polynomial. It can be mathematically definedin a short way, and can be built, characterized, particularized, or refined in severalways.
For instance, we get a bijection between bounded regions (bipolar orientations in thecase of a graph) and bases with internal activity one and external activity zero, whichcan be seen as an elaboration on linear programming. We get a decomposition of intobounded regions of minors of the primal and the dual, which also has a counterpart fordecomposing matroid bases, and yields an expression of the Tutte polynomial usingonly beta invariants of minors. We also get activity preserving bijections between allreorientations and all subsets (related to a four-variable expansion of the Tuttepolynomial), between regions and no-broken-circuit subsets (acyclic case), betweenincreasing trees and permutations (complete graph case)...
It is the subject of a series of paper.
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The active bijection:short mathematical definition
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The active bijection (in one slide) Aim of the talk: explain this slide!
For every oriented matroid M on a linearly ordered set E ,α(M) is the basis of M defined by the three following properties:
I If M is acyclic and min(E ) is contained in every positivecocircuit of M, then α(M) is the unique (fully optimal) basisB of M such that:
I for all b ∈ M \min(E ), the signs of b and min(C∗(B; b))are opposite in C∗(B; b);
I for all e 6∈ B, the signs of e and min(C (B; e))are opposite in C (B; e).
I α(M∗) = E \ α(M)
I α(M) = α(M/F ) ] α(M(F ))where F is the [complementary of the] union of all positive [co]circuits ofM whose smallest element is the greatest possible smallestelement of a positive [co]circuit of M; [...]=equivalent dual formulation
The mapping α yields an activity preserving bijection:- between all activity classes of reorientations and all bases of M,- and between all reorientations and all subsets of M.
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The active bijection in graphsFor every directed graph
−→G on a linearly ordered set of edges E ,
α(−→G ) is the spanning tree of G defined by:I If−→G is acyclic and min(E ) is contained in every directed
cocycle of−→G , then α(
−→G ) is the unique (fully optimal)
spanning tree B of G such that:I for all b ∈ E \min(E ), the signs of b and min(C∗(B; b))
are opposite in C∗(B; b);I for all e 6∈ B, the signs of e and min(C (B; e))
are opposite in C (B; e).
I α(M∗) = E \ α(M) If−→G is strongly connected and min(E ) is
contained in every directed cycle, then... [dual formulation]
I α(−→G ) = α(
−→G /F ) ] α(
−→G (F )) where F can be the
[complementary of the] union of all directed [co]cycles of−→G whose
smallest element is the greatest possible smallest element of a
directed [co]cycle of−→G ; [...]= equivalent required dual formulation
The mapping α yields an activity preserving bijection:- between all activity classes of orientations and all spanning trees of G ,
- and between all orientations and all subsets of G .5/ 61
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Graphs, hyperplane arrangements,and oriented matroids
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Hyperplane arrangement −→ oriented matroid
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_ 2_ 3_ 4_ 56
_2_3456 23456
_2 _3 _4 _5 _6 23
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12345_6
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_2_3_4_5_6
Matroid: incidence properties and flat intersection lattice
Oriented matroid: convexity properties and face relative positions
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Directed graph −→ associated arrangement
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edge ij −→ hyperplane with equation vj − vi = 0spanning tree −→ basis
orientation of edge ij −→ half-space vj − vi >0directed cut −→ vertex of the region (positive cocircuit)
cut −→ vertex (cocircuit)acyclic orientation −→ region
strongly connected orientation −→ region of the dual arrangement8/ 61
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Duality
Every oriented matroid M has a dual M∗.
circuits of M = cocircuits of M∗
cocircuits of M = circuits of M∗
acyclic orientations of M = totally cyclic orientations of M∗
(or regions) (or dual regions)(or strongly connected orientations)
bases of M = complementary of bases of M∗
In the realizable case: duality ∼ orthogonality
In the graphical case: duality = cycles/cocycles duality(extends planar graph duality)
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Arrangements of pseudolines ∼ Rank 3 oriented matroids
c©[Oriented Matroids 1998] reference book
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Oriented matroids
Combinatorial axiomaticsCircuits (or cocircuits) are signed subsets satisfying simplecombinatorial axioms...
(There are about 20 known equivalent combinatorial axiomatics!)
Topological representation theorem [Folkman & Lawrence 1978]
Oriented matroids on E∼
Signed pseudosphere arrangements (Se)e∈E up to homeomorphism
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The Tutte polynomialin oriented matroidsand directed graphs
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Bipolar orientations and bounded regions
graph−→ hyperplane arrangement
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bipolar orientations w.r.t. p = 1: −→ bounded regions w.r.t. p = 1:acyclic orientations regions that do not touch p = 1with unique source and unique sinkextremities of p = 1
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The β invariant
M underlying matroid
I β(M) = # bounded regions w.r.t. e (on one side of e)
I β(M) = # bipolar orientations w.r.t. e(for a given orientation of e)
I β(M) does not depend on e: it is an invariant
I β(M) = t1,0(M) coefficient of x (or y) of theTutte polynomial tM(x , y) of M
I β(M) = # acyclic (re)orientations such that e belongs toevery positive cocircuit (directed cocycle)Other coefficients of tM can also be interpreted a similar way...
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Activities of orientations
Let M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical results
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Activities of orientations
Let M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical results
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Activities of orientations
Let M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical results
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Activities of orientationsLet M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical resultso1,0 = # bounded regions = #bipolar orientations w.r.t. e ∈ E(with given orientation for e)
[Crapo 1969, Zaslavsky 1975, Las Vergnas 1977]15/ 61
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Activities of orientationsLet M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical resultst(M; 2, 0) = # regions = # acyclic orientations
[Winder 1966, Stanley 1973, Las Vergnas 1977]
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Activities of orientationsLet M be an oriented matroid on a linearly ordered set E
(or a directed graph−→G = (V ,E )).
I An element of E is active if it is the smallest of a positivecircuit of M (or: ... the smallest edge of a directed cycle)
I An element of E is dual-active if it is the smallest of a positivecocircuit of M (or: ... the smallest edge of a directed cocycle)
Theorem [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j2i+j
x i y j
where oi ,j is the number of reorientations of M with i dual-activeelements and j active elements.
Related classical resultst(M; 0, 2) = # dual regions = # totally cyclic orientations
(strongly connected orientations)
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Dual activity of a region (or an acyclic orientation)
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Consider the smallest element of each cocircuitThey correspond to the sequence of nested faces induced by theminimal basis: 1 ∩ 2 ⊂ 1The grey region (digraph) has dual-active elements: 124
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Dual activity of a region (or an acyclic orientation)
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t(K4; x , y) = 8.( x2 )3 + +12.( x2 )2 + 4.( x2 ) + . . .t(K4; 2, 0) = 24
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Activities of bases (spanning trees)Let M be a matroid on a linearly ordered set E
(or a graph G = (V ,E )).
B a basis (spanning tree) of M
I e ∈ E \ B is externally active w.r.t. B if it is the smallestelement of Ce = C (B; e), the unique circuit (cycle) containedin B ∪ {e}
I b ∈ B is internally active w.r.t. B if it is the smallest elementof C ∗b = C ∗(B; b), the unique cocircuit (cocycle) contained in(E \ B) ∪ {b}
Theorem [Tutte 1954 & Crapo 1969]
t(M; x , y) =∑i ,j
bi ,j x i y j
ou bi ,j is the number of bases of M with i internally activeelements and j externally active elements.
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The active bijection in oriented matroids
Tutte polynomial t(M; x , y) of an ordered oriented matroid M
Theorem. [Tutte 1954 & Crapo 1969]
t(M; x , y) =∑i ,j
bi ,jxiy j
ou bi ,j = number of bases with activities (i , j)
Theorem. [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j
(x2
)i(y2
)jwhere oi ,j = number of reorientations with activities (i , j)
oi ,j = 2i+jbi ,j
The active bijection is a canonical underlying bijection...
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3
The fully optimal basis(The fully optimal spanning tree)
of a bounded region(of a bipolar orientation)
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M an oriented matroid on a linearly ordered set
E = e1 < ... < en
We look for a bijection between
bounded regions −AM w.r.t. e1
o∗(−AM) = 1 and o(−AM) = 0
and
(1,0)-active bases B of Mι(B) = 1 and ε(B) = 0
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Fully Optimal Basis (in an oriented matroid)
M an oriented matroid on a linearly ordered set
E = e1 < ... < en
B ⊆ E a basis of M
Ce = fundamental circuit of e 6∈ B w.r.t. B= unique circuit in B ∪ e
C ∗b = fundamental cocircuit of b ∈ B w.r.t. B= unique cocircuit in (E \ B) ∪ b
The basis B is fully optimal if
I b and min C ∗b have opposite signs in C ∗b for all b ∈ B \ e1
I e and min Ce have opposite signs in Ce for all e ∈ E \ B
Remarkif M has a fully optimal basis B then M is bounded w.r.t. e1
and B is (1, 0)-active 21/ 61
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Active bijection: main theorem in the bounded case
I An ordered bounded oriented matroid w.r.t. min(E ) M has aunique fully optimal basis denoted
α(M)
I The mapping α is a bijection between (pairs of opposite)
bounded regions of M and (1, 0)-active bases of M.
Computational remarks
I The fully optimal basis α(M) is difficult to compute (the problemcontains the real/combinatorial linear programming problem).
I But the unique reorientation α−1(B) associated with the basis B is
easy to compute (just sign the elements from the first to the last so
that the criterion is satisfied).
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Active bijection: main theorem in the bounded case
I An ordered bounded oriented matroid w.r.t. min(E ) M has aunique fully optimal basis denoted
α(M)
I The mapping α is a bijection between (pairs of opposite)
bounded regions of M and (1, 0)-active bases of M.
Computational remarks
I The fully optimal basis α(M) is difficult to compute (the problemcontains the real/combinatorial linear programming problem).
I But the unique reorientation α−1(B) associated with the basis B is
easy to compute (just sign the elements from the first to the last so
that the criterion is satisfied).
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Active bijection: main theorem in the bounded case
I An ordered bounded oriented matroid w.r.t. min(E ) M has aunique fully optimal basis denoted
α(M)
I The mapping α is a bijection between (pairs of opposite)
bounded regions of M and (1, 0)-active bases of M.
Computational remarks
I The fully optimal basis α(M) is difficult to compute (the problemcontains the real/combinatorial linear programming problem).
I But the unique reorientation α−1(B) associated with the basis B is
easy to compute (just sign the elements from the first to the last so
that the criterion is satisfied).
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Ex: 136 is the fully optimal basis of the green region.
1
2 3
4 5
6
2
2
3
3
4
4
5
5 6
6
1
1
124
456
2345
C∗1 2 C∗3 4 5 C∗61 +C2 + – –3 +C4 + – – –C5 + – +6 +
(fundamental tableau of the basis)24/ 61
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Usual Linear Programming Optimal Bases (simplex criterion)
p=1
p=1
f=2
f=2
3
3
4
4 5
5
6
6
7
7
R
- + 137
1 2 3 4 5 6 7
1
2
3
4
5
6
7
++
+++
_
+__
+
__
_
_
++_
+
145
1 2 3 4 5 6 7
1
2
3
4
5
6
7
+++
+
__
_
++
_
+
+_
++_
__
147
1 2 3 4 5 6 7
1
2
3
4
5
6
7
+++
+
__ _
++_
__
_
+
__
+
157
1 2 3 4 5 6 7
1
2
3
4
5
6
7
+++
+
__
__
++_ _
_
++
_
+
Only takes the first column and first row into account.
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The Fully Optimal Basis
1
1
2
2
3
3
4
4 5
5
6
6
7
7
137
1 2 3 4 5 6 7
1
2
3
4
5
6
7
++
+++
_
+__
+
__
_
_
++_
+
147
1 2 3 4 5 6 7
1
2
3
4
5
6
7
+++
+
__ _
++_
__
_
+
__
+
157
1 2 3 4 5 6 7
1
2
3
4
5
6
7
+++
+
__
__
++_ _
_
++
_
+
Takes all columns and rows into account.
Two elaborations on linear programming:- multiobjective programming: unique optimal vertex- flag programming: unique sequence of nested faces (induction)
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A “Bijective Characterization” of LP OptimalityD
C
B
A
9
8
7
6
5
f =423f =21
p=1
13C
137
138
13A14A
14C
147
148
139 149158
168
16B136135
13B
157
15D 15A
16A
169
167
16D
159
In the real case: the active bijection is the unique bijection associating(1, 0)-active flags and bounded regions with the adjacency property.
In the graphical case: multiobjective LP ∼ a cocycle weight functionIn the general case: we need the dual adjacency property,
and it implies the bijection.27/ 61
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Duality for bounded regions (bipolar orientations)
Bounded region M w.r.t. e ←−−−−−−−→canonical bijection
Dual-bounded region M∗
o∗(M) = 1, o(M) = 0 o∗(M∗) = 0, o(M∗) = 1
idem ←−−−−−−−→canonical bijection
Dual-bounded region −eMo∗(−eM) = 0, o(−eM) = 1
e=1
2 3
6 5
4
s t ←−−−−−−−→canonical bijection
e=1
2 3
6 5
4
s t
Bipolar orientation−→G Cyclic-bipolar orientation −e
−→G
Strong duality property (refines LP duality):
α(−eM) = α(M) \ {e} ∪ {e ′}
for M bounded w.r.t. e, where E = e < e ′ < . . ..(means that the active bijection is compatible with the above bijections) 28/ 61
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4
Decompositions of activitiesand Tutte polynomial
in terms of beta invariants of minors
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Active bijection: inductive decomposition construction
For every oriented matroid M on a linearly ordered set E ,α(M) defined by the three following properties:
I α(M) is the fully optimal basis of M if it is a bounded region
I α(M∗) = E \ α(M)
I α(M) = α(M/F ) ] α(M(F ))where F is the union of all positive circuits of M whosesmallest element is the greatest possible active element of M
Lemma. If F 6= ∅, then M(F ) is dual-bounded, and M/F has oneless active element than M.
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Active bijection: inductive decomposition construction
For every oriented matroid M on a linearly ordered set E ,α(M) defined by the three following properties:
I α(M) is the fully optimal basis of M if it is a bounded region
I α(M∗) = E \ α(M)
I α(M) = α(M/F ) ] α(M(F ))where F is the complementary of the union of all positivecocircuits of M whose smallest element is the greatestpossible dual-active element of M;
(equivalent dual formulation)
Lemma. If F 6= ∅, then M/F is bounded, and M(F ) has one lessdual-active element than M.
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Active bijection: direct decomposition constructionLet M be an ordered oriented matroid on Ewith ι = o∗(M) dual-active elements a1 < ... < aιand ε = o(M) active elements a′1 < ... < a′ε.
The active decomposing sequence of M is
∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
I It corresponds to the active partition of M:
E = F ′ε−1 \ F ′ε ] . . .] F ′0 \ F ′1 ] F1 \ F0] . . .] Fι \ Fι−1
I For 0 ≤ k ≤ ε− 1, we have F ′k =⋃
D positive circuitMin D>a′k
D.
I Dually, for 0 ≤ k ≤ ι− 1, we have Fk = E \⋃
D positive cocircuitMin D>ak
D.
I Fc is the union of all positive circuits of M (directed cycles), andE \ Fc is the union of all positive cocircuits (directed cocycles).
Fc is a cyclic flat.
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Active bijection: direct decomposition constructionLet M be an ordered oriented matroid on Ewith ι = o∗(M) dual-active elements a1 < ... < aιand ε = o(M) active elements a′1 < ... < a′ε.
The active decomposing sequence of M is
∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
I It corresponds to the active partition of M:
E = F ′ε−1 \ F ′ε ] . . .] F ′0 \ F ′1 ] F1 \ F0] . . .] Fι \ Fι−1
I For 0 ≤ k ≤ ε− 1, we have F ′k =⋃
D positive circuitMin D>a′k
D.
I Dually, for 0 ≤ k ≤ ι− 1, we have Fk = E \⋃
D positive cocircuitMin D>ak
D.
I Fc is the union of all positive circuits of M (directed cycles), andE \ Fc is the union of all positive cocircuits (directed cocycles).
Fc is a cyclic flat.
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Active bijection: direct decomposition constructionLet M be an ordered oriented matroid on Ewith ι = o∗(M) dual-active elements a1 < ... < aιand ε = o(M) active elements a′1 < ... < a′ε.
The active decomposing sequence of M is
∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
I It corresponds to the active partition of M:
E = F ′ε−1 \ F ′ε ] . . .] F ′0 \ F ′1 ] F1 \ F0] . . .] Fι \ Fι−1
I for 1 ≤ k ≤ ι, the minor M(Fk)/Fk−1 is dual-bounded (cyclic-bipolar)
I for 1 ≤ k ≤ ε, the minor M(F ′k−1)/F ′k is bounded (bipolar)
Direct definition of α:
α(M) =⊎
1≤k≤ια(M(Fk)/Fk−1) ]
⊎1≤k≤ε
α(M(F ′k−1)/F ′k)
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Example
46 5 13 21
3 2
6 5
4
t 1
3 2
6 5
4
1
3 2
6 5
4
1
3 2
6 5
4
2
2
3
3
4
4
5
5 6
6
1
1
2...
4...
1...
1...2...
2...
1...
1...
4...
2...
14
14
Dual-active elements: 1 4Active partition: 123 ] 456
Active decomposing sequence: ∅ ⊂ 123 ⊂ 123456Bounded (bipolar) minors : M(123) and M/123
α(−→G ) = 134
Activity class:−→G , −123
−→G , −456
−→G , −123456
−→G .
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Reorientation activity classes of ordered oriented matroidsActivity class of M: the set of 2o(M)+o∗(M) reorientations of Mobtained by reorienting independently the o(M) + o∗(M) parts ofthe active partition of M (all its elements have same active partition)
They are interesting on their own:
I Activity classes form a partition of the set 2E of reorientations of M
I Refine the acyclic / totally cyclic usual decomposition
I Describe the intersections of regions w.r.t. a given (minimal) flag
I Wide generalization of components of acyclic orientations of graphsdefined on particular linear orderings of the vertices, as related to
I Non-commutative monoids [Cartier & Foata 1969]
I Heaps of pieces [Viennot 1986]
I Acyclic orientations and the chromatic polynomial [Lass 2001]
I There is one and only one unique sink acyclic orientation in eachactivity class of acyclic orientation of a graph, as related to
t(G ; 1, 0) = #acyclic orientations of G with unique sink
[Zaslavsky 1975, Greene & Zaslavsky 1983, Gebhard & Sagan 2000]34/ 61
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Example of active partitions of rank 4 regions
1 2
3
d
b
c
a
f
g
e
h
4
1 + 2 + 3 + 4abcdefg
4
1 + 2 + 34abcdefgh
h
341 + 2abcdefgh +
e
34 2abcdh1efg + +
g
34 2abcdh1efg + +
f
341 + 2abcdefgh +
a
341 + 2abcdefgh +
c
34 2aefgh1bcd + +
b
34 2aefgh1bcd + +
d
1 + 2 + 3 + 4abcdefg
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Active bijection main theorem
I α(M) is a basis of M
I active (resp. dual-active) elements of Mare externally (resp. internally) active elements of α(M)
I more precisely, α also preserves active partitions(they exist for bases too and ensure that α(M) is a basis)
I the basis α(M) is also associated with the 2o(M)+o∗(M)
reorientations in the activity class of M
I α is a bijection between all activity classes of reorientations ofM and all bases of M.
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Active bijection between region activity classes and internal bases
D
C
B
A
9
8
7
6
5
f =423f =21
p=1
13C
137
138
13A14A
14C
147
148
139 149158
168
16B136135
13B
157
15D 15A
16A
169
167
16D
159
13414D
124125
12C
126
129
127
12A
12B
128
124
14D134
124
124
128
125
12A
126
12B
12C
127
129
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Decomposing sequencesLet E be a finite linearly ordered set.
We call abstract decomposing sequence of E a sequence of subsetsof E such that:
I ∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
I the sequence min(Fk \ Fk−1), 1 ≤ k ≤ ι is increasing with k
I the sequence min(F ′k−1 \ F ′k), 1 ≤ k ≤ ε, is increasing with k
Let M be a matroid on E .
A decomposing sequence of M is an abstract decomposingsequence of E such that:
I for every 1 ≤ k ≤ ι, the minor M(Fk)/Fk−1 is either a singleisthmus, or is connected (2-connected for a graph)
I for every 1 ≤ k ≤ ε, the minor M(F ′k−1)/F ′k is either a singleloop, or is connected (2-connected for a graph)
Nota bene. For |E | > 1, M connected (2-connected) if and only if β(M) 6= 038/ 61
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Theorem:decomposing oriented matroids into bounded regions
Let M be an oriented matroid on a linearly ordered set E .
2E = {reorientations A ⊆ E of M}
=⊎{
A ⊆ E | −AM(Fk)/Fk−1, 1 ≤ k ≤ ι, bounded w.r.t. min(Fk\Fk−1),
and −AM(F ′k−1)/F ′k , 1 ≤ k ≤ ε, dual-bounded w.r.t. min(F ′k−1\F ′k)
}where the disjoint union is over all decomposing sequences of M∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
(the decomposing sequence of M associated in the second term to areorientation A is the active decomposing sequence of −AM.)
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Theorem:decomposing matroid bases into (1, 0)-activity bases
Let M be a matroid on a linearly ordered set E .
{bases of M} =⊎
∅=F ′ε⊂...⊂F ′0=Fc
Fc=F0⊂...⊂Fι=Edecomposing sequence of M
{B ′1]...]B ′ε]B1]...]Bι |
for all 1 ≤ k ≤ ε, B ′k base of M(F ′k−1)/F ′k with ι(B ′k) = 0 and ε(B ′k) = 1,
for all 1 ≤ k ≤ ι, Bk base of M(Fk)/Fk−1 with ι(Bk) = 1 and ε(Bk) = 0}
Then B = B ′1 ] ... ] B ′ε ] B1 ] ... ] Bι it the active partition of B and
Int(B) = ∪1≤k≤ιmin(Fk \ Fk−1) = ∪1≤k≤ιInt(Bk),
Ext(B) = ∪1≤k≤εmin(F ′k−1 \ F ′k) = ∪1≤k≤εExt(B ′k).
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Theorem:Tutte polynomial in terms of β invariants of minors
Let M be a matroid on a linearly ordered set E .
t(M; x , y) =∑ (∏
1≤k≤ι β(M(Fk)/Fk−1
))(∏1≤k≤ε β
′(M(F ′k−1)/F ′k))
x ι y ε
where β′(M) = β(M) if |E | > 1, β′(isthmus) = 0, and β′(loop) = 1
and where the sum can be equally:
I either over all decomposing sequences of M
I or over all abstract decomposing sequences of E
∅ = F ′ε ⊂ ... ⊂ F ′0 = Fc = F0 ⊂ ... ⊂ Fι = E
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Classical results refined by this formulaTheorem. [Tutte 1954]
t(M; x , y) =∑i ,j
bi ,jxiy j
where bi ,j = # (i , j)-active bases
Theorem. [Las Vergnas 1984]
t(M; x , y) =∑i ,j
oi ,j(x
2)i(y
2)j
where oi ,j = # (i , j)-active reorientations
Theorem. [Etienne & Las Vergnas 1998, Kook Reiner & Stanton 1999]
t(M; x , y) =∑
t(M/F ; x , 0) t(M(F ); 0, y)
where the sum can be equally either over all subsets F of E , orover all cyclic flats F of M.
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5
Further results
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Refined bijection between (re)orientations and subsetsLet M be an (oriented) matroid on a linearly ordered set E .
Partition of 2E in terms of reorientation activity classes
2E =⊎
activity classes of M(one −AM chosen in each class)
{2o(−AM)+o∗(−AM) reorientations obtained by
active partition reorienting
}
Partition of 2E in terms of matroid basis activity intervals
[Crapo 1969, Dawson 1981, ...]
2E =⊎
B basis of M
[B \ Int(B), B ∪ Ext(B)
]
Active bijectionOne activity class ←→ one basis ←→ one interval(2i+j elements) (2i+j elements)
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Refined bijection between (re)orientations and subsets
Active bijectionOne activity class ←→ one basis ←→ one interval(2i+j elements) (2i+j elements)
The activity class of −AM and the matroid basis interval ofB = α(−AM) are isomorphic boolean lattices.
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Refined bijection between (re)orientations and subsets
Refined active bijection
For A ⊆ E and B = α(−AM), set
α∅M(A) = B \ (A ∩ Int(B)) ∪ (A ∩ Ext(B))
Theorem
I α∅M is a bijection between 2E (reorientations) and 2E (subsets)
I restricted to acyclic reorientations, α∅M is a bijection betweenregions (acyclic orientations) and no-broken-circuit subsets(i.e. subsets of bases with external activity zero)
I it preserves activities, active partitions, and also somefour-variable refined activities
(that take into acount the positions in the boolean lattices)
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Refined bijection between (re)orientations and subsets
Refined active bijection
For A ⊆ E and B = α(−AM), set
α∅M(A) = B \ (A ∩ Int(B)) ∪ (A ∩ Ext(B))
Theorem
I α∅M is a bijection between 2E (reorientations) and 2E (subsets)
I restricted to acyclic reorientations, α∅M is a bijection betweenregions (acyclic orientations) and no-broken-circuit subsets(i.e. subsets of bases with external activity zero)
I it preserves activities, active partitions, and also somefour-variable refined activities
(that take into acount the positions in the boolean lattices)
Remark. α∅M depends on M and applies to A (on the contrary with α).
The ∅ symbol is a parameter that can be changed to get other similar
refined active bijections, with other boolean lattice bijections. 46/ 61
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Active bijection between regions and no-broken-circuit subsets
(acyclic restriction of the refined active bijection w.r.t. the grey region)
D
C
B
A
9
8
7
6
5
f =423f =21
p=1
13C
137
138
13A14A
14C
147
148
139 149158
168
16B136135
13B
157
15D 15A
16A
169
167
16D
159
13414D
1 4 (2)
1 5 (2)
1 C (2)
1 6 (2)
1 9 (2)
1 7 (2)
1 A (2)
1 B (2)
1 8 (2)
1 (24)
1 D (4) 13 (4)
12 (4)
124
128
125
12A
126
12B
12C
127
129
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Refined basis activitiesLet M be a matroid on a linearly ordered set E . Let B ⊆ E be abasis of M. Let A be in the boolean interval[B \ IntM(B),B ∪ ExtM(B)]. Set:
IntM(A) = IntM(B) ∩ A;
PM(A) = IntM(B) \ A;
ExtM(A) = ExtM(B) \ A;
QM(A) = ExtM(B) ∩ A.
Theorem. [Gordon & Traldi 1990, Las Vergnas 2013]
Let M be a matroid on a linearly ordered set E .
T (M; x + u, y + v) =∑A⊆E
x |IntM(A)|u|PM(A)|y |ExtM(A)|v |QM(A)|
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Refined orientation activitiesLet M be an oriented matroid on a linearly ordered set E . LetA ⊆ E . Set:
ΘM(A) = O(−AM) \ A, θM(A) = |ΘM(A)|,ΘM(A) = ΘM(E \ A) = O(−AM) ∩ A, θM(A) = |ΘM(A)|.
Θ∗M(A) = ΘM∗(A) = O∗(−AM) \ A, θ∗M(A) = |Θ∗M(A)|,ΘM(A) = ΘM∗(A) = O∗(−AM) ∩ A, θ∗M(A) = |Θ∗M(A)|.
Theorem.
T (M; x + u, y + v) =∑A⊆E
xθ∗M(A)uθ
∗M(A)yθM(A)v θM(A).
Proof: by the partition of 2E into activity classes.49/ 61
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Refined bijection between (re)orientations and subsets
For B = α(−AM),
α∅M(A) = B \ (A ∩ Int(B)) ∪ (A ∩ Ext(B))
Theorem (continued)I α∅M is a bijection between 2E (reorientations) and 2E (subsets)I It preserves the four refined activities, i.e. for all A ⊆ E :
IntM(α∅M(A)) = Θ∗M(A)
PM(α∅M(A)) = Θ∗M(A)
ExtM(α∅M(A)) = ΘM(A)
QM(α∅M(A)) = ΘM(A)
(bijection for the equality of the two expressions of t(M; x + u, y + v))
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And now for something completely different
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Deletion/contraction construction of various bijections
M oriented matroid on a linearly ordered set E with max(E ) = ω.
Choice at each step:{α(M), α(−ωM)
}={α(M \ ω), α(M/ω) ∪ ω
}Theorem.With suitable choices, we get whole classes of bijections betweenbases: all / subsets / internal / no broken circuit subsets / ...andreorientations: classes / all / specific / acyclic / ...
Various properties can be demanded: activities / adjacency / ...
Specifying choices yield: THE active bijection.
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Deletion/contraction construction of the active bijectionM oriented matroid on a linearly ordered set E with max(E ) = ω.
Choice at each step:{α(M), α(−ωM)
}={α(M \ ω), α(M/ω) ∪ ω
}One way of defining the active bijection:
I If M is a region (acyclic orientation):I If M is not cut by ω then α(M) = α(M \ ω)
(and α(−ωM) = α(M/ω) ∪ ω)I If M is cut by ω then −ωM is also a region and
I If M and −ωM do not have same active partition then thechoice is determined
I Otherwise then compare the flags associated to each region inthe bounded minor containing ω in order to preserve the fullyoptimal basis adjacency property
(refines the usual LP induction by variable/constraint deletion)
I If M is a dual-region, then use dual rules.I If M is not a region nor a dual-region, then use the same rules in
region/dual-region minors (can be done also directly)
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Supersolvable arrangement (generalizes chordal graph)
Inductive construction: at each new dimension, the newhyperplanes cut each other in lower dimension hyperplanes
(roughly said)
1
2 3 4
5
6
7
8
9
1
2 3 4
1
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Supersolvable arrangement (generalizes chordal graph)
Inductive construction: at each new dimension, the newhyperplanes cut each other in lower dimension hyperplanes
(roughly said)
1 2 3 4
5
6
7
8
9
Allows a construction in nested “fibers” dimension by dimension,that fits well with the deletion/contraction construction...
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Signed permutations
Regions of the hyperoctahedral arrangement ∼ Signed permutations(it is supersolvable) ∼ Cube symmetries
∼ Coxeter group Bn
1_
1 1_
2 2_
2 12
2_
3
1_
3
3_
3
13
23
1 2_
2
3
_
3
3_
21
2_
31
_
213
_
21_
3
_
2_
31_
3_
21
31_
2
13_
2
1_
23
1_
2_
3
1_
3_
2
_
31_
2
312
132
123
12_
3
1_
32
_
312
321
231
213
21_
3
2_
31_
321
No-broken-circuit base subsets ∼ edge-signed increasing forests
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Active bijection between signed permutations and edge-signed increasing forests
1
2
3
123_1_2_3
1
2 3
132_1_3_2
1 2
3
213
2_1_3_
213_2_1_3
1 2
3
231
23_1_
2_31_
2_3_1
1
2
3
312
3_1_2_
312_3_1_2
1 2 3
321
32_1
3_21
3_2_1_
321_32
_1_
3_21_
3_2_1
1
2
3
12_3
_1_23
1
2
3
1_23
_12
_3
1
2
3
1_2_3
_123
1
2 3
13_2
_1_32
1
2 3
1_32
_13
_2
1
2 3
1_3_2
_132
1 2
3
21_3
2_13_
21_3_
2_13
1 2
3
2_31
2_3_1_
23_1_
231
1
2
3
31_2
3_12_
31_2_
3_12
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Permutations (acyclic complete graph case)
Regions of the braid arrangement ∼ Permutations(it is supersolvable) ∼ Simplex symmetries
∼ Coxeter group An
∼ Acyclic orientations of Kn+1
12 13 23
14
24
34
3124
3142
3412
4312
1324
1342
1432
4132
1234
1243
1423
4123
No-broken-circuit base subsets ∼ increasing trees 57/ 61
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Active bijection between permutations and increasing trees
1
2
3
4
123
14322341
1
2
3 4
132
13422431
1
2 3
4
213
1423241331423241
1
2 3
4
231
1243214334123421
1
2
3
4
312
1324231441324231
1
2 3 4
321
12342134312432144123421343124321
It is equal to a classical bijection!
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oriented matroidsactivity classes of reorientations basesact. cl. of acyclic reorientations bases B with ε(B) = 0
act. cl. of totally cyclic reor. bases B with ι(B) = 0bounded acyclic reorientations bases B with ι(B) = 1 and ε(B) = 0
reorientations subsetsacyclic reorientations no-broken-circuit subsets
totally cyclic reorientations supsets of bases B with ι(B) = 0hyperplane arrangements
reorientations ∼ signatures bases ∼ simplicesacyclic reorientations ∼ regions
graphsreorientations ∼ orientations bases ∼ spanning trees
unique sink acyclic orientations spanning trees B with ε(B) = 0bipolar orientations sp. trees B with ι(B) = 1 and ε(B) = 0
source-sink reversed bipolar orientations sp. trees B with ι(B) = 0 and ε(B) = 1uniform oriented matroids
bounded regions linear programming optimal verticesbraid arrangement or complete graph or Coxeter arrangement An
permutations increasing treeshyperoctahedral arrangement or Coxeter arrangement Bn
signed permutations signed increasing trees59/ 61
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6
References
60/ 61
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)
• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
61/ 61
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.
• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
61/ 61
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.
• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)
• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)
• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)
• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)
• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).
• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
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References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!
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![Page 80: The Active Bijection - tutte2015.ma.rhul.ac.uktutte2015.ma.rhul.ac.uk/files/2015/02/Tutte-workshop-2015-Gioan-A... · The active bijection (in one slide) Aim of the talk: explain](https://reader033.vdocuments.net/reader033/viewer/2022060513/5f82062f730f92422b54da7a/html5/thumbnails/80.jpg)
References• Las Vergnas. A correspondence between spanning trees and orientations in graphsGraph Theory and Combinatorics, Academic Press, London, UK, 233-238, (1984)• G. Correspondance naturelle entre bases et reorientations des matroıdes orientes.Ph.D. Universite Bordeaux 1, 2002.• G.-L.V. Bases, reorientations, and linear programming, in uniform and rank 3oriented matroids. Advances in Applied Mathematics 32, 212–238 (2004)• G.-L.V. Activity preserving bijections between spanning trees and orientations ingraphs. Discrete Mathematics, Vol. 298, pp. 169-188,(2005)• G.-L.V. The active bijection between regions and simplices in supersolvablearrangements of hyperplanes. Electronic. J. Comb., Vol. 11(2), pp. 39,(2006)• G.-L.V. Fully optimal bases and the active bijection in graphs, hyperplanearrangements, and oriented matroids. EuroComb’07 (Sevilla), ENDM 365-371 (2007)• G.-L.V. A Linear Programming Construction of Fully Optimal Bases in Graphs andHyperplane Arrangements. EuroComb’09 (Bordeaux), ENDM 307-311 (2009)• G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids - 1 - The fully optimal basis of a bounded region. European Journal ofCombinatorics, Vol. 30 (8), pp. 1868-1886 (2009)• G.-L.V. The active bijection in graphs: overview, new results, complements andTutte polynomial expressions.
(almost) ready for submission and available on demand or at Arxiv (2015).• • • G.-L.V. The active bijection in graphs, hyperplane arrangements, and orientedmatroids. 2. Decomposition of activities. / 3. Linear programming construction offully optimal bases. / 4. Deletion/contraction constructions and universality.Back in active preparation...
THANKS!61/ 61