Recovering locally compact spaces from disjointness
relations on function algebras
Luiz G. Cordeiro46th COSy
University of Ottawa
June 7, 2018
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 1 / 23
Motivation
Theorem (Gelfand-Naimark '43)
If X and Y are compact Hausdor� and T : C (X )→ C (Y ) is a∗-isomorphism, then X ' Y .
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 2 / 23
Motivation
Generalizations: We can recover X from C (X ) (or C (X ,R)) as
Banach-Stone '37
A Banach space: ‖f ‖∞ = sup |f |(X ).
Gelfand-Kolmogorov '39
A ring: (f + g)(x) = f (x) + g(x), (fg)(x) = f (x)g(x)
Milgram '40
A multiplicative semigroup: (fg)(x) = f (x)g(x).
Kaplansky '47
A lattice: f ≤ g ⇐⇒ ∀x(f (x) ≤ g(x)).
+other recent results, even for non scalar functions.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 3 / 23
Motivation
Proof of Gelfand-Naimark
. Open sets Closed ideals
U {f : f = 0 outside U}
= {f : supp(f ) ⊆ U}
points Maximal ideals /
General proof of the other results
. Open sets �ideals�
U {f : supp(f ) ⊆ U}
points Maximal ideals /
and Urysohn's Lemma repeatedly.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 4 / 23
Goals
Develop general techniques to recover all these results in fullgenerality;
Classify isomorphisms for di�erent algebraic structures;
Non-scalar valued functions;
Locally compact spaces;
Non-commutative setting.
Ingredients
Supports;
Urysohn's Lemma.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 5 / 23
De�nitions and notation
Convention:
X ,Y , . . . will be locally compact Hausdor� (the domains).
CX ,CY , . . . will be Hausdor� (the codomains).
Let θ : X → CX be a �xed continuous function (the zero).
De�nition
[f 6= θ] = {x : f (x) 6= θ(x)};supp(f ) = [f 6= θ];
Cc(X ,CX ) = {f : X → CX : supp(f ) is compact}.
Example
If CX = R (or C), θ = 0.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 6 / 23
De�nitions and notation
Example
If CX is a group, θ = 1.
Example
If CX is a semigroup with a zero, θ = 0.
Example
If C is an lattice with minimum, θ = minC.
Example
If CX = X , θ = idX .
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 7 / 23
De�nitions and notation
De�nition (Urysohn's property)
If A ⊆ Cc(X ,CX ) is a subset containing θ. We say that A is regular if forall x ∈ X , Uopen 3 x and c ∈ CX , there exists f ∈ A such that f (x) = cand supp(f ) ⊆ U.
Convention: A(X ) will be always regular for some CX and θ.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 8 / 23
Some relations
De�nition1 f ⊥ g : if [f 6= θ] ∩ [g 6= θ] = ∅ (f and g are weakly disjoint);2 f ⊥⊥ g : if supp f ∩ supp g = ∅ (f and g are strongly disjoint);
supp(f ) supp(g) supp(f ) supp(g)
f ⊥ g f ⊥⊥ g
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 9 / 23
The main theorem
Theorem
If T : A(X )→ A(Y ) is a ⊥⊥-isomorphism then there is a unique
homeomorphism φ : Y → X such that φ(suppTf ) = supp f for all
f ∈ A(X ).
Proof. Appropriate notion of ⊥⊥-ideal:
Uopen ←→ I(U) = {f ∈ A : supp(f ) ⊆ U}
De�nition
φ is the T -homeomorphism.
Why ⊥⊥ and not ⊥?The result is false for ⊥(weak)-isomorphisms.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 10 / 23
Recovering results
Let C = R, θ = 0.
Example (Milgram)
Let Cc(X ,R) as a multiplicative semigroup. Then
f ⊥ g ⇐⇒ fg = 0, the absorbing element
f ⊥⊥ g ⇐⇒ ∃h(hf = f and h ⊥ g)
(same for C, [0, 1], D1,. . . ).
f
g
f
gh
f ⊥ g f ⊥⊥ g
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 11 / 23
Basic ⊥⊥-isomorphisms
How to classify an �isomorphism� T : Cc(X ,CX )→ Cc(Y ,CY )?
φ : Y → X homeomorphism: Tf = f φ;
χ : CX → CY homeo/isomorphism: Tf = χ(f φ);
χ : Y × CX → CY such that sections χ(y , ·) are homeo/isomorphisms:
Tf (y) = χ(y , f φ(y)).
De�nition
T : A(X )→ A(Y ) is basic if there are such χ and φ.
χ is called the T -transform.
Proposition
T is basic i� f (φy) = g(φy) ⇐⇒ Tf (y) = Tg(y).
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 12 / 23
Basic ⊥⊥-isomorphisms and their transforms
Theorem
T : Cc(X ,CX )→ Cc(Y ,CY ) is a basic ⊥⊥-isomorphism, and CX and CY
are �good enough� (e.g. admit some Lie group structure). TFAE
(1) Each section χ(y , ·) is a continuous;
(2) T is continuous with respect to the topologies of pointwise
convergence.
(3) χ is continuous.
Proposition
If CX and CY have some operation ∗ and A(X ) and A(Y ) are (pointwise)
∗-closed, then a basic T : A(X )→ A(Y ) is a ∗-morphism i� every section
χ(y , ·) is a ∗-morphism.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 13 / 23
Example of a T -transform
C = R, θ = 0.
Example
If T : Cc(X ,R)→ Cc(Y ,R) is basic ⊥⊥-isomorphism
Tf (y) = χ(y , f (φ(y)))
T is linear ⇐⇒ χ(y , ·) is linear for all y⇐⇒ χ(y , t) = p(y)t for some p(y)
⇐⇒ Tf (y) = p(y)f (φ(y))
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 14 / 23
Consequences
For X locally compact Hausdor�, we recover X from Cc(X ,R) (orCc(X ,C)) from
Linear ‖ · ‖∞-isometries (Banach-Stone `37);
Multiplicative isomorphisms (Milgram `40 ⊇ Gelfand-Kolmogorov `39);
Lattice isomorphisms (Kaplansky `47);
Linear ⊥-preserving isomorphisms (Jarosz `90);
�Compatibility order�-isomorphisms (Kania-Rmoutil `16).
and for X compact and C (X ):
Linear non-vanishing isomorphisms (Li-Wong `14);
Non-vanishing group isomorphisms (Hernández-Ródenas `07)
+ classi�cations of isomorphisms for all except Kania-Rmoutil.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 15 / 23
New consequences
Endow C (X , S1) with the supremum metric:
d(f , g) = supx∈X|f (x)− g(x)|.
Theorem
If X and Y are Stone spaces and T : C (X ,S1)→ C (Y , S1) is an isometric
isomorphism, then there is a homeomorphism φ : Y → X and a continuous
function p : Y → {±1} such that Tf (y) = f (φy)p(y)
Rewording
Every isometric isomorphism between unitary groups of commutative unitalC*-algebras of real rank zero extends to an isomorphism orconjugate-isomorphism on complementary corners.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 16 / 23
Groupoids
A groupoid is a small category with inverses.
Example
If a group G acts on a set X , the transformation groupoid
G n X = {(gx , g , x) : x ∈ X , g ∈ G}
with product
(z , h, y)(y , g , x) = (z , hg , x) x y zg
hg
h
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 17 / 23
Groupoid algebras
Discrete case
CG = 〈δg : δgδh = δgh if sensible, 0 o/w〉.
Continuous case: C ∗r (G) is de�ned in terms of a Haar system. (Renault &Anantharaman-Delaroche)
Example
Jiang-Su algebra Z; Razak-Jacelon algebra W; graph/higher rank graphalgebras; Cuntz-Krieger; group algebras; C (X )-crossed products; Kirchberg.
Xin Li (arXiv:1802.01190 [math.OA]): classi�able unital stably �niteC*-algebras admit (twisted) groupoid models.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 18 / 23
New consequences
Let G be a locally compact Hausdor� groupoid with a regular fullysupported Haar system λG and µG a fully supported regular Borel measureon G(0). De�ne the fully supported measure λG ⊗ µG by∫
G
fd(λG ⊗ µG) =∫G(0)
(∫Gx
f (g)dλxG(g)
)dµG(x)
Theorem
If T : Cc(G)→ Cc(H) is an algebra isomorphism which is
(λZ ⊗ µZ )-isometric (Z = G,H), then there are unique topological
groupoid isomorphism φ : H→ G and a continuous cocycle p : H→ S1such that
Tf (h) = p(h)D(φ(h))f (φ(h))
where D(g) =dλ
r(g)G
d(φ∗λ
φ−1(r(g)H
)(g), and in this case µG = φ∗µH.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 19 / 23
New consequences
Similar results hold for:
Étale Haar groupoids (G, λ), with norm
‖f ‖I ,r = supx∈G(0)
∫Gx
|f |dλx , f ∈ Cc(G)
and diagonal (Cc(G(0)))-preserving ‖ · ‖I ,r -isometric isomorphisms
For topologically principal, ample groupoids G, and diagonal-preservingisomorphisms of Steinberg algebras AR(G) over indecomposable ringsR .
Thank you.
Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 20 / 23
References I
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Luiz Cordeiro (uOttawa) Disjoint functions June 7, 2018 21 / 23
References II
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References III
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[16] James G. Wendel, On isometric isomorphism of group algebras,Paci�c J. Math. 1 (1951), 305�311. MR 0049910
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