Hung-Chih LeeDepartment of Information Technology Ling Tung University

Maximum packings and minimum coverings of

multigraphs with paths and stars

Chun-Cheng ChenDepartment of Mathematics National Central University

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Outline

Introduction

Packing and covering of λKn

Packing and covering of λKn,n

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Introduction

Kn : the complete graph with n vertices.

Km,n : the complete bipartite graph with parts of sizes m and n.If m = n, the complete bipartite graph is referred to as balanced.

Sk (k-star) : the complete bipartite graph K1,k.

Pk (k-path) : a path on k vertices.

Ck (k-cycle) : a cycle of length k.

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λH : the multigraph obtained from H by replacing each edge e by λ edges each having the same endpoints as e.When λ = 1, 1H is simply written as H.

A decomposition of H : a set of edge-disjoint subgraphs of H whose union is H.

A G-decomposition of H : a decomposition of H in which each subgraph is isomorphic to G.

If H has a G-decomposition, we say that H is G-decomposable and write G|H.

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Example.

P4|K6

K6 K3,4

S3|K3,4

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An (F, G)-decomposition of H : a decomposition of H into copies of F and G using at least one of each.

If H has an (F, G)-decomposition, we say that H is (F, G)-decomposable and write (F, G)|H.

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Example.

(P4, S3)|K6

K6 K3,4

(P4, S3)|K3,4

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Abueida and Daven [4] investigated the problem of the (C4, E2)-decomposition of several graph products where E2 denotes two vertex disjoint edges.[4] A. Abueida and M. Daven, Multidecompositions of several graph products, Graphs Combin. 29 (2013), 315-326.

Abueida and Daven [3] investigated the problem of (Kk, Sk)-decomposition of the complete graph Kn.[3] A. Abueida and M. Daven, Multidecompositons of the complete graph, Ars Combin. 72(2004), 17-22 .

Abueida and O‘Neil [7] settled the existence problem for (Ck, Sk-1)-decomposition of the complete multigraph λKn for k {3,4,5}.∈[7] A. Abueida and T. O'Neil, Multidecomposition of λKm into small cycles and claws, Bull. Inst. Combin. Appl. 49 (2007), 32-40.

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Priyadharsini and Muthusamy [15, 16] gave necessary and sufficient conditions for the existence of (Gn,Hn)-decompositions of λKn and λKn,n where Gn,Hn ∈ {Cn,Pn,Sn-1}.[15] H. M. Priyadharsini and A. Muthusamy, (Gm,Hm)-multifactorization of λKm, J. Com-bin. Math. Combin. Comput. 69 (2009), 145-150.

[16] H. M. Priyadharsini and A. Muthusamy, (Gm,Hm)-multidecomposition of Km,m(λ),Bull. Inst. Combin. Appl. 66 (2012), 42-48.

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Abueida and Daven [2] and Abueida, Daven and Roblee [5] completely determined the values of n for which λKn admits a (G,H)-decomposition where (G,H) is a graph-pair of order 4 or 5.[2] A. Abueida and M. Daven, Multidesigns for graph-pairs of order 4 and 5, Graphs Com-bin. 19 (2003), 433-447.[5] A. Abueida, M. Daven, and K. J. Roblee, Multidesigns of the λ-fold complete graph for graph-pairs of order 4 and 5, Australas J. Combin. 32 (2005), 125-136.

A graph-pair (G,H) of order m is a pair of non-isomorphic graphs G and H on m non-isolated vertices such that GH is isomorphic to Km.

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Shyu [17] investigated the problem of decomposing Kn into paths and stars with k edges, giving a necessary and sufficient condition for k = 3.[17] T.-W. Shyu, Decomposition of complete graphs into paths and stars, Discrete Math. 310 (2010), 2164-2169.

Abueida, Clark and Leach [1] and Abueida and Hampson [6] considered the existence ofdecompositions of Kn−F for the graph-pair of order 4 and 5, respectively, where F is a Hamiltonian cycle, a 1-factor, or almost 1-factor.[1] A. Abueida, S. Clark, and D. Leach, Multidecomposition of the complete graph into graph pairs of order 4 with various leaves, Ars Combin. 93 (2009), 403-407.[6] A. Abueida and C. Hampson, Multidecomposition of Kn − F into graph-pairs of order 5 where F is a Hamilton cycle or an (almost) 1-factor, Ars Combin. 97 (2010), 399-416

Shyu [20] investigated the problem of decomposing Kn into cycles and stars with k edges, settling the case k = 4.[20] T.-W. Shyu, Decomposition of complete graphs into cycles and stars, Graphs Combin. 29 (2013), 301-313.

In [18, 19], Shyu considered the existence of a decomposition of Kn into paths and cycles with k edges, giving a necessary and sufficient condition for k { ∈ 3, 4}.[18] T.-W. Shyu, Decompositions of complete graphs into paths and cycles, Ars Combin. 97(2010), 257-270.[19] T.-W. Shyu, Decomposition of complete graphs into paths of length three and triangles, Ars Combin. 107 (2012), 209-224.

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Lee [12] and Lee and Lin [13] established necessary and sufficient conditions for the existence of (Ck,Sk)-decompositions of the complete bipartite graph and the complete bipartite graph with a 1-factor removed, respectively.[12] H.-C. Lee, Multidecompositions of complete bipartite graphs into cycles and stars, Ars Combin. 108 (2013), 355-364.

[13] H.-C. Lee and J.-J. Lin, Decomposition of the complete bipartite graph with a 1- factor removed into cycles and stars, Discrete Math. 313 (2013), 2354-2358.

In [21], Shyu considered the existence of a decomposition of Km,n into paths and stars with k edges, giving a necessary and sufficient condition for k = 3.[21] T.-W. Shyu, Decomposition of complete bipartite graphs into paths and stars with same number of edges, Discrete Math. 313 (2013), 865-871.

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When a multigraph H does not admit an (F,G)-decomposition, two natural questions arise :

(1) What is the minimum number of edges needed to be removed from the edge set of H so that the resulting graph is (F,G)-decomposable?

(2) What is the minimum number of edges needed to be added to the edge set of H so that the resulting graph is (F,G)-decomposable?

These questions are respectively called the maximum packing problem and the minimum covering problem of H with F and G.

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Let F, G, and H be multigraphs. For subgraphs L and R of H.An (F,G)-packing ((F,G)-covering) of H with leave L (padding R) is an (F,G)-decomposition of H−E(L) (H+E(R)).An (F,G)-packing ((F,G)-covering) of H with the largest (smallest) cardinality is a maximum (F,G)-packing (minimum (F,G)-covering) of H, and its cardinality is the (F,G)-packing number ((F,G)-covering number) of H, denoted by p(H;F,G) (c(H;F,G)).

An (F,G)-decomposition of H is a maximum (F,G)-packing (minimum (F,G)-covering) with leave (padding) the empty graph.

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Example.

(P5, S4)|K6 –E(P4)K6

(P5, S4)|K6

+E(P2)

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Abueida and Daven [2] and Abueida, Daven and Roblee [5] gave the maximum packing and the minimum covering of Kn and λKn with G and H, respectively, where (G,H) is a graph-pair of order 4 or 5.[2] A. Abueida and M. Daven, Multidesigns for graph-pairs of order 4 and 5, Graphs Com-bin. 19 (2003), 433-447.

[5] A. Abueida, M. Daven, and K. J. Roblee, Multidesigns of the λ-fold complete graph for graph-pairs of order 4 and 5, Australas J. Combin. 32 (2005), 125-136.

Abueida and Daven [3] obtained the maximum packing and the minimum covering of the complete graph Kn with (Kk,Sk).[3] A. Abueida and M. Daven, Multidecompositons of the complete graph, Ars Combin. 72(2004), 17-22 .

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Let G be a multigraph.

The degree of a vertex x of G (degG(x)) : the number of edges incident with x.

The center of Sk : the vertex of degree k in Sk.The endvertex of Sk : the vertex of degree 1 in Sk.

(x;y1,y2,…,yk) : the Sk with center x and endvertices y1,y2,…,yk.

Packing and covering of λKn

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v1v2… vk : the Pk through vertices v1,v2,…,vk in order, and the vertices v1 and vk are referred to as its origin and terminus.

If P = x1x2…xt, Q = y1y2…ys and xt = y1, then P + Q = x1x2…xty2…ys.

Pk(v1,vk) : a Pk with origin v1 and terminus vk.

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G[U] : the subgraph of G induced by U.G[U,W] : the maximal bipartite subgraph of G with bipartition (U,W).

G1G2…Gt or : the graph with vertex set and edge set .

When the edge sets are disjoint, G =expresses the decomposition of G into G1,G2,…,Gt.

When G1,G2,…,Gt are multigraphs, not necessarily disjoint.

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[9] Darryn Bryant, Packing paths in complete graphs, J. Combin. Theory Ser. B 100 (2010), 206-215.

Proposition 2.1.For positive integers λ, n, and t, and any sequence m1,m2,…,mt of positive integers, the complete multigraph λKn can be decomposed into paths of lengths m1,m2,…,mt if and only if mi ≤ n-1 andm1+m2+…+mt = |E(λKn)|.

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[8] J. Bosák, Decompositions of Graphs, Kluwer, Dordrecht, Netherlands, 1990.

[10] P. Hell and A. Rosa, Graph decompositions, handcuffed prisoners and balanced P-designs, Discrete Math. 2 (1972), 229-252.

Proposition 2.2.For a positive even integer n and V (Kn) = {x0, x1,…, xn-1}, the complete graph Kn can be decomposed into copies of n-paths Pn(x0,x), Pn(x1,x1+ ) ,…, Pn(x1, xn-1).

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Theorem 2.5.Let n and k be positive integers and let t be a nonnegative integer with k ≥ 3,n ≥ k + 2, and t ≤ k-1. If |E(Kn)| ≡ t (mod k), then Kn has a (Pk+1, Sk)-packing with leave Pt+1.

Proof.

Let n = qk+r and |E(Kn)| = pk+t with integers q,p,r and 0 ≤ r ≤ k -1.

Now we consider the case r ≠ 1, we set n = m + sk + 1 where m and s are positive integers with 1 ≤ m ≤ k-1.

Case 1. m is odd.

Case 2. m is even.

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Case 2. m is even.

By Proposition 2.2, Km=Pm.

By Proposition 2.1 and Lemma 2.4, Ksk+1=Pk-m (p-ms-)Pk+1Pt+1.

Kn = Km+sk+1 = KmKm,sk+1Ksk+1.

Hence, Kn = msSk(p-ms)Pk+1Pt+1.

Ksk+1

Km

Sk|K1,sk

…

Pk+1

P

Sk|K1,sk

……

P

Proposition 2.2.For a positive even integer n and V (Kn) = {x0,x1,…,xn-1}, the complete graph Kn can be decomposed into copies of n-paths Pn(x0,x), Pn(x1,x1+ ) ,…, Pn(x1,xn-1).

Lemma 2.4.Let k, m, p, and s be positive integers and let t be a nonnegative integer with max{m,t} ≤ k-1.If thenp-(s+1)m > 0.

Proposition 2.1.

For positive integers λ, n, and t, and any sequence m1,m2,…,mt of positive integers, the complete multigraph λKn can be decomposed into paths of lengths m1,m2,…,mt if and only if mi ≤ n-1 and m1+ m2+…+mt = |E(λKn)|.

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Theorem 2.6.

Let λ, n and k be positive integers and let t be a nonnegative integer with k ≥ 3, n ≥ k+2, and t ≤ k-1. If |E(λKn)| ≡ t (mod k), then λKn has a (Pk+1, Sk)-packing with leave Pt+1.

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Corollary 2.7.

For positive integers λ, k, and n with k ≥ 3, the complete multigraph λKn is (Pk+1,Sk)-decomposable if and only if n ≥ k + 1, ≡ 0 (mod k) and (λ,n) ≠ (1, k + 1).

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For positive integers k, n, and t with k ≤ n-1 and t ≤ k-1.

If λKn has a (Pk+1, Sk)-packing with leave Pt+1, then it has a (Pk+1, Sk)-covering with padding Pk-t+1.

Since p(λKn; Pk+1,Sk) ≤ ≤ ≤ c(λKn; Pk+1,Sk), we have the following by Theorem 2.6.

Theorem 2.8.

If λ, n, and k are positive integers with n ≥ k+2 ≥ 5, then p(λKn; Pk+1,Sk) =and c(λKn; Pk+1, Sk) =.

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Packing and covering of λKn,n

(v1,v2,…,vk) : the k-cycle of G through verticesv1,v2,…,vk in order.

μ(uv) : the number of edges of G joining u and v.

A central function c from V(G) to the set of non-negative integers is defined as follows : for each v ∈V(G), c(v) is the number of Sk in the decomposition whose center is v.

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Proposition 3.1.

For a positive integer k, a multigraph H has an Sk-decomposition with central function c if and only if let c : V → N {0}.(i) ;(ii) for all ;(iii) for all , .where denotes the number of edges of H with both ends in S.

[11] D.G. Hoffman, The real truth about star designs, Discrete Math. 284 (2004), 177-180.

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Lemma 3.4.

Let λ, k, and n be positive integers with 3 ≤ k < n < 2k. If , then λKn,n has a (Pk+1, Sk)-packing P with |P|=and a (Pk+1, Sk)-covering C with |C|=.

Lemma 3.6.

Let λ, k, and n be positive integers with 3 ≤ k < n < 2k. If , then λKn,n has a (Pk+1, Sk)-packing P with |P|=and a (Pk+1, Sk)-covering C with |C|=.

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Let n = k + r. The assumption k < n < 2k implies 0 < r < k.

λKn,n = λKk,k λKk,r λKr,k λKr,r

𝜆𝑟 2=𝜆 (𝑛−𝑘)2<𝑘

Proof of Lemma 3.4.

Packing.

By Lemma 3.3.⇒ (Pk+1, Sk) |Kk,k

≅ λ𝑟 𝑆𝑘

leave

Lemma 3.3.If λ and k are positive integers with k ≥ 2, then λKk,k is (Pk+1, Sk)-decomposable.

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Let , , and

Define a (k + 1)-path P as follows:

P

Let P’ be the (k-s+1)-subpath of P with origin for odd k and with origin for even k.

… … … …

… … … …𝑏0 𝑏1 𝑏2 𝑏

⌊ 𝑠−12

⌋𝑏𝑛−1𝑏𝑘− 1

𝑏⌈ 𝑠−12

⌉𝑏⌊ 𝑠−12

⌋−1… … … …𝑏

⌊ 𝑘− 12

⌋𝑏⌈ 𝑘−12

⌉𝑏⌊ 𝑘− 12

⌋− 1

… … … …𝐴0 𝐴1

𝐵1𝐵0… … … …

𝑎0 𝑎1 𝑎2 𝑎⌊ 𝑠− 12

⌋𝑎𝑛−1𝑎𝑘−1

𝑎⌈ 𝑠−12

⌉𝑎⌊ 𝑠− 12

⌋− 1… … … …𝑎

⌊ 𝑘−12

⌋𝑎⌈ 𝑘− 12

⌉𝑎⌊ 𝑘−12

⌋ −1

H = λKn,n - E(P) + E(P’).

PP’

Covering Let s =. Claim. λKn,n+E(Pk-s+1)=Pk+1Sk.

k is odd.

s is odd.

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Proposition 3.1.

For a positive integer k, a multigraph H has an Sk-decomposition with central function c if and only if let c : V → N{0}.(i) ;(ii) for all ;(iii) for all , .where denotes the number of edges of H with both ends in S.

Claim. Sk|H

Define a function c : V (H)→N{0} as follows :

𝑐 (𝑣 )={ 0 𝑖𝑓 𝑣∈𝐵0 ,   λ h𝑜𝑡 𝑒𝑟𝑤𝑖𝑠𝑒 .

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Theorem 3.7.

If λ, k, and n are positive integers with 3 ≤ k ≤ n, then p(λKn,n; Pk+1, Sk) = and c(λKn,n; Pk+1, Sk ) = .

Corollary 3.8.

For positive integers λ, k and n with k≥3, the balanced complete bipartite multigraph λKn,n is (Pk+1, Sk)-decomposable if and only if k ≤ n and is divisible by k.

[1] A. Abueida, S. Clark, and D. Leach, Multidecomposition of the complete graph into graph pairs of order 4 with various leaves, Ars Combin. 93 (2009), 403-407.

[2] A. Abueida and M. Daven, Multidesigns for graph-pairs of order 4 and 5, Graphs Com-bin. 19 (2003), 433-447.

[3] A. Abueida and M. Daven, Multidecompositons of the complete graph, Ars Combin. 72(2004), 17-22 .

[4] A. Abueida and M. Daven, Multidecompositions of several graph products, Graphs Combin. 29 (2013), 315-326.

[5] A. Abueida, M. Daven, and K. J. Roblee, Multidesigns of the λ-fold complete graph for graph-pairs of order 4 and 5, Australas J. Combin. 32 (2005), 125-136.

[6] A. Abueida and C. Hampson, Multidecomposition of Kn − F into graph-pairs of order 5 where F is a Hamilton cycle or an (almost) 1-factor, Ars Combin. 97 (2010), 399-416

[7] A. Abueida and T. O'Neil, Multidecomposition of λKm into small cycles and claws, Bull. Inst. Combin. Appl. 49 (2007), 32-40.

[8] J. Bosák, Decompositions of Graphs, Kluwer, Dordrecht, Netherlands, 1990.

[9] Darryn Bryant, Packing paths in complete graphs, J. Combin. Theory Ser. B 100(2010), 206-215.

[10] P. Hell and A. Rosa, Graph decompositions, handcuffed prisoners and balanced P-designs, Discrete Math. 2 (1972), 229-252.

[11] D.G. Hoffman, The real truth about star designs, Discrete Math. 284 (2004), 177-180.

[12] H.-C. Lee, Multidecompositions of complete bipartite graphs into cycles and stars, Ars Combin. 108 (2013), 355-364.

[13] H.-C. Lee and J.-J. Lin, Decomposition of the complete bipartite graph with a 1-factor removed into cycles and stars, Discrete Math. 313 (2013), 2354-2358.

[14] C. A. Parker, Complete bipartite graph path decompositions, Ph.D. Thesis, Auburn Uni-versity, Auburn, Alabama, 1998.

[15] H. M. Priyadharsini and A. Muthusamy, (Gm,Hm)-multifactorization of λKm, J. Com-bin. Math. Combin. Comput. 69 (2009), 145-150.

[16] H. M. Priyadharsini and A. Muthusamy, (Gm,Hm)-multidecomposition of Km,m(λ),Bull. Inst. Combin. Appl. 66 (2012), 42-48.

[17] T.-W. Shyu, Decomposition of complete graphs into paths and stars, Discrete Math. 310 (2010), 2164-2169.

[18] T.-W. Shyu, Decompositions of complete graphs into paths and cycles, Ars Combin. 97(2010), 257-270.

[19] T.-W. Shyu, Decomposition of complete graphs into paths of length three and triangles, Ars Combin. 107 (2012), 209-224.

[20] T.-W. Shyu, Decomposition of complete graphs into cycles and stars, Graphs Combin. 29 (2013), 301-313.

[21] T.-W. Shyu, Decomposition of complete bipartite graphs into paths and stars with same number of edges, Discrete Math. 313 (2013), 865-871.

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