A list version of graph packing

0.37 MB·English

by  Ervin Győri

A LIST VERSION OF GRAPH PACKING
ERVIN GYO˝RI, ALEXANDR KOSTOCHKA, ANDREW MCCONVEY, AND DERREK YAGER
Abstract. We consider the following generalization of graph packing. Let G = (V ,E )
1 1 1
andG =(V ,E )begraphsofordernandG =(V V ,E )abipartitegraph. Abijection
2 2 2 3 1 2 3
∪
f fromV ontoV isalistpacking ofthetriple(G ,G ,G )ifuv E impliesf(u)f(v) / E
1 2 1 2 3 2 2
∈ ∈
5 and vf(v) / E for all v V . We extend the classical results of Sauer and Spencer and
3 1
1 Bollob´as an∈d Eldridge on∈packing of graphs with small sizes or maximum degrees to the
0
settingoflistpacking. Inparticular,weextendthewell-knownBollob´as–EldridgeTheorem,
2
provingthatif∆(G ) n 2,∆(G ) n 2,∆(G ) n 1,and E + E + E 2n 3,
1 2 3 1 2 3
n then either (G ,G ,G≤) p−acks or is o≤ne o−f 7 possible≤exc−eptions.|Ho|pef|ully|, t|he c|o≤ncep−t of
a 1 2 3
list packing will help to solve some problems on ordinary graph packing, as the concept of
J
list coloring did for ordinary coloring.
1
1

] Mathematics Subject Classification: 05C70, 05C35.
O
Keywords: Graph packing, maximum degree, edge sum, list coloring.
C
. Introduction
h 1.
t
a
The notion of graph packing is a well-known concept in graph theory and combinatorics.
m
Two graphs on n vertices are said to pack if there is an edge-disjoint placement of the graphs
[
onto the same set of vertices. In 1978, two seminal papers, [6] and [1], on extremal problems

1
on graph packing appeared in the same journal. In particular, Sauer and Spencer [6] proved
v
8 sufficient conditions for packing two graphs with bounded product of maximum degrees.
8
Theorem 1 ([6]). Let G and G be two graphs of order n. If 2∆(G )∆(G ) < n, then G
4 1 2 1 2 1
2 and G pack.
2
0
. This result is sharp and later Kaul and Kostochka [5] characterized all graphs in which
1
0 Theorem 1 is sharp.
5
Theorem 2 ([5]). Let 2∆(G )∆(G ) n. G and G do not pack if and only if one of G
1 1 2 1 2 1
≤
v: and G2 is a perfect matching and the other is either Kn2,n2 with n2 odd or contains Kn2+1.
i
X In the same paper, Sauer and Spencer gave sufficient conditions for packing two graphs
r with given total number of edges.
a
Theorem 3 ([6]). Let G and G be two graphs of order n. If E(G ) + E(G ) 3n 2,
1 2 | 1 | | 2 | ≤ 2 −
then G and G pack.
1 2
This result is best possible, since G = K and G = nK do not pack. Independently,
1 1,n−1 2 2 2
Bolloba´s and Eldridge [1] proved the stronger result that the bound of Theorem 3 can be
significantly strengthened when ∆(G ) < n 1 and ∆(G ) < n 1.
1 2
− −
Research of this author is supported in part by OTKA Grants 78439 and 101536.
Research of this author is supported in part by NSF grant DMS-1266016 and by grants 12-01-00631 and
12-01-00448 of the Russian Foundation for Basic Research.
The author acknowledges support from National Science Foundation grant DMS 08-38434 “EMSW21-
MCTP: Research Experience for Graduate Students.”
1
Theorem 4 ([1]). If ∆(G ),∆(G ) n 2,e(G )+e(G ) 2n 3, and G ,G is not one of
1 2 1 2 1 2
(cid:83) ≤ −(cid:83) (cid:83) ≤ − (cid:83){ }(cid:83)
the following pairs: 2K ,K K , K K ,K K , 3K ,K K , K K ,2K ,
2 1 3 2 3 2 3 2 2 4 3 3 3
(cid:83) (cid:83) { (cid:83) } (cid:83){ (cid:83) } { } { }
2K K ,K K , K K ,K 2K , K K ,3K (Figure 1). Then, G and G
2 3 3 4 4 4 2 3 5 4 3 1 2
{ } { } { }
pack.
G(2) H(2)
G(1) H(1)
x x y y x1 x2 y1 y2 G(5) H(5) G(6) H(6)
1 2 1 2 x3 y3 x1 x2 y1 y2 x1 x2 y1 y2
x y y x x y y
3 3 4 3 4 3 4
x x y y x x y y
4 3 3 4 4 5 4 5 x x y x x y
4 5 5 5 6 6
y
5
x x y y x x y y
6 7 7 6 7 8 7 8
G(3) H(3) G(4) H(4)
x x y y x x y y
1 2 1 2 1 2 1 2
G(7) H(7)
x3 x4 y3 y4 x3 x4 y3 y4 x1 x2 x5 x6 y1 y4 y5 y7
x7
x5 x6 y5 y6 x5 x6 y5 y6 x3 x4 x8 x9 y2 y3y6 y8 y9
Figure 1.
Bad pairs in Theorems 4 and 6.
This result is also sharp, since the graphs G = C and G = K K satisfy the
1 n 2 1,n−2 1
∪
maximum degree condition, have 2n 2 edges, and do not pack. There are other extremal
−
examples.
Variants of the packing problem have been studied and, in particular, restrictions of per-
missible packings arise both within proofs and are posed as independent questions. The
notion of a bipartite packing was introduced by Catlin [2] and was later studied by Ha-
jnal and Szegedy [4]. This variation of traditional packing involves two bipartite graphs
G = (X Y ,E ) and G = (X Y ,E ) where permissible packings send X onto X
1 1 1 1 2 2 2 2 1 2
∪ ∪
and Y onto Y . The problem of fixed-point free embeddings, studied by Schuster in 1978,
1 2
considers a different restriction to the original packing problem [7]. In this case, two edge
disjoint copies of a graph G are placed into K with the additional property that two copies
n
of the same vertex must be mapped to different vertices in K . In [9], Schuster’s result is
n
used to prove a necessary condition for packing two graphs with given maximum and average
degrees.
In this paper, we introduce the language of list packing in order to model such problems.
A list packing of the graph triple (G ,G ,G ) with G = (V ,E ),G = (V ,E ), and G =
1 2 3 1 1 1 2 2 2 3
(cid:83)
(V V ,E ) is a bijection f : V V such that uv E implies f(u)f(v) / E and for
1 2 3 1 2 1 2
→ ∈ ∈
each u V , uf(u) / E . Note that both G and G are graphs on n vertices so that G
1 3 1 2 3
∈ ∈
has 2n vertices, and one can think of the edge set E as a list of restrictions that must be
3
avoided when packing G and G .
1 2
This notion is closely related to Vizing’s concept of list coloring [8]. Suppose we wish to
color a graph G with the colors 1,...,k . A list assignment L is a function on the vertex
{ }
set V(G) that returns a set of colors L(v) 1,...,k not permissible for v. A list coloring,
⊂ { }
more specifically an L-coloring, is a proper coloring f of G such that f(v) / L(v) for all
∈
v V(G). In fact, the problem of list coloring G can be stated within the framework of
∈
list packing. A proper L-coloring of a graph G is equivalent to a list packing where G = G
1
along with an appropriate number of isolated vertices, G is a disjoint union of K ’s each
2 n
2
representing a color, and E consists of all edges going between a vertex v V and the
3 1
∈
copies of K corresponding to colors in L(v).
n
Similarly, the restrictions to packings discussed above can be modeled using this frame-
work. A bipartite packing is a packing of the triple (G ,G ,G ) where E consists of all
1 2 3 3
edges between X and Y for i = 1,2. A fixed-point free embedding is a packing of the
i 3−i
triple (G,G,G ) where E = (v,v) : v V(G) . Although the list packing is more general,
3 3
{ ∈ }
some important theorems on the ordinary packing can be transferred to the list setting. The
results of this paper prove natural generalizations of Theorems 1–4 in the language of list
packing. In particular, we extend Theorem 1 and Theorem 2 as follows.
Theorem 5. Let G = (G ,G ,G ) be a graph triple with V = V = n. If ∆(G )∆(G )+
1 2 3 1 2 1 2
| | | |
∆(G ) n/2, then G does not pack if and only if ∆(G ) = 0 and one of G or G is
3 3 1 2
≤
a perfect matching and the other is Kn2,n2 with n2 odd or contains Kn2+1. Consequently, if
∆(G )∆(G )+∆(G ) < n/2, then G packs.
1 2 3
The main result of this paper is the following list version of Theorem 4.
Theorem 6. Let n 1 and G and G be n-vertex graphs. If ∆(G ),∆(G ) n 2,
1 2 1 2
≥ ≤ −
∆(G ) n 1, E(G ) + E(G ) + E(G ) 2n 3 and the pair (G ,G ) is none of the
3 1 2 3 1 2
≤ − | | | | | | ≤ −
7 pairs in Figure 1, then G and G pack.
1 2
Theorem 6 is sharp and the list version introduces several new sharpness examples. First,
the condition ∆ n 1 cannot be removed, since a vertex in V adjacent to all vertices
3 1
≤ −
in V cannot be placed at all (Figure 2A). The restriction on the edge sum is also sharp, as
2
there are several examples of graphs with E > 0 and edge sum equal to 2n 2 that do not
3
−
pack. We provide 4 such examples.
x y x y x y
1 1 1 1 1 1
x y x y x y
2 2 2 2 2 2
x y x y x y
3 3 3 3 3 3
x y x y x y
n 2 n 1 n 2 n 2 n 2 n 2
− − − − − −
x y x y x y
n 1 k+2 n 1 n 1 n 1 n 1
− − − − −
x y x y x y
n n n n n n
(a) (b) (c)
x1 yn x1 y1
x2 yn−1 x2 y2
x
3 x
3
y
x ym0+1 x4 yk
m m0 y
k+1
xxmm++12 ym0−1 xn−2 yk+2
x
y n 1
2 −
xn y1 xn yn
(e)
(d)
Figure 2.
Sharpness examples for Theorem 6
For the first example, consider G and G to be independent sets and x ,x V each
1 2 1 2 1
∈
adjacent to all but a single vertex of V (Figure 2B). Alternatively, consider E consisting of a
2 1
3
single edge x x , E consisting of a single edge y y , and E consisting of all edges between
1 2 2 n−1 n 3
x ,x andV y y (Figure2C).Forthethirdexample, considerG = K K ,
1 2 2 n−1 n 1 1,m−1 n−m
{ } − − ∪
G = K K (for any choice of m,m(cid:48)), and E consisting of all yellow edges between
2 1,m(cid:48)−1 n−m(cid:48) 3
∪
the center of the star in G and isolated vertices in V as well as between the center of the
1 2
star in G and isolated vertices in V (Figure 2D). Finally, consider G = K K ,
2 1 1 1,n−1 1
∪
G = C K (for any choice of k), and let E consist of all possible edges between the
2 k n−k 3
∪
center of the star in G and isolated vertices in G (Figure 2E).
1 2
Though this paper focuses on extending classical packing results to the list setting, one of
our goals is to provide tools to handle problems of standard graph packings. In particular,
we heavily use Theorems 5 and 6 in [3] to get an approximate solution to a conjecture of
˙
Zak [9] on packing n-vertex graphs with given sizes and maximum degrees.
The paper is organized as follows. In the next paragraph, we introduce some notation. In
Section 2, we prove Theorem 5. Section 3 contains some preliminary results, including an
extension of Theorem 3 that will be used as a base case in our proof of Theorem 6. Then,
Section 4 contains our proof of the main result by induction on the size of the vertex set.
1.1. Notation. A graph triple G = (G ,G ,G ) of size n consists of a pair of n-vertex
1 2 2
graphs G = (V ,E ) and G = (V ,E ) together with a bipartite graph G = (V V ,E ).
1 1 1 2 1 2 3 1 2 3
∪
Let V := V V . An edge in E E is a white edge, while an edge in E is a yellow edge.
1 2 1 2 3
∪ ∪
For v V (i = 1,2), the white neighborhood of v, denoted N (v) V , is the set of neighbors
i i i
∈ ⊆
of v in G , d (v) = N (v) , and ∆ = max d (v). For convenience, when w V , we say
i i | i | i v∈Vi i ∈ 3−i
that N (w) = (and hence d (w) = 0). The yellow neighborhood of v, denoted N (v) V
i i 3 3−i
∅ ⊆
is the set of neighbors of v in G and d (v) = N (v) . For v V , the neighborhood in v,
3 3 3 i
| | ∈
denoted N(v) is the disjoint union N (v)+N (v) and the degree of v is d (v)+d (v) and is
i 3 i 3
denoted d(v).
For i = 1,2,3, let e = E and define ∆ to be the max d (v). Finally, the triple G
i i i v∈V i
| |
packs if there is a bijection f : V V such that vf(v) / E for any v V and uv E
1 2 3 1 1
→ ∈ ∈ ∈
implies f(u)f(v) / E .
2
∈
Proof of Theorem 5
2.
( ) Suppose G1 is a perfect matching. If G2 contains Kn+1, then for any mapping f : V1
⇐ 2 →
V2, some edge of G1 will be mapped to an edge in the clique. Otherwise, G2 is Kn,n with
n odd, then under any mapping, we are again forced to have some matching edg2e2in G
2 1
mapped so that it has one endpoint in each partite set.
( ) Assume that our graph triple G is the minimal counterexample that does not pack
⇒
where we interpret minimal as the minimal number of total edges. If ∆ = 0, then the result
3
follows from Theorem 2. Hence, we can assume E = . By minimality, we may assume
3
(cid:54) ∅
that there is a partial packing f which has a conflict at only a single edge in vw E ,
3
∈
where f(v) = w. For an arbitrary a V v with f(a) = b, define the mapping f by
1 a
∈ −
f (v) = b,f (a) = w and f = f otherwise. In particular, f will be a packing of the graph
a a a a
triple G if a satisfies:
(i) f (N (a)) N (w) = ,
a 1 2
∩ ∅
(ii) f (N (v)) N (b) = ,
a 1 2
∩ ∅
(iii) b / N (v), and
3
∈
(iv) w / N (a)
3
∈
4
Note that there are at most ∆ ∆ vertices in V v that may violate (i) [similarly for (ii)]
1 2 1
−
and at most ∆ 1 vertices in V v that may violate (iii) [similarly for (iv)]. Since G
3 1
− −
does not pack, (n 1) [(∆ 1) + (∆ 1) + 2∆ ∆ ] 0. But this inequality yields
3 3 1 2
n+1 2[∆ +∆ ∆− ], a−contra−diction. (cid:3)− ≤
3 1 2
≤
Preliminary facts
3.
The following lemma is an extension of Theorem 3.
Lemma 7. Let G = (V ,E ) and G = (V ,E ) be graphs of order n and let G = (V
1 1 1 2 2 2 3 1
∪
V ,E ) be a bipartite graph with partite sets V and V . If d (v) n 1 for each v V V
2 3 1 2 3 1 2
(cid:4) (cid:5) ≤ − ∈ ∪
and e +e +e 3n 2, then the triple G = (G ,G ,G ) packs.
1 2 3 ≤ 2 − 1 2 3
Proof: If e = 0, then the result holds from Theorem 3. Further, if e = 0 for i 1,2 ,
3 i
∈ { }
then the problem reduces to finding a matching in G which can be done by Hall’s Theorem.
3
So we assume that e ,e ,e > 0.
1 2 3
(cid:4) (cid:5)
It is sufficient to prove the case when e + e + e = 3n 2. The proof will proceed
1 2 3 2 −
by induction on n. If n = 2, then e + e + e = 1 and it is clear that there is a packing.
1 2 3
Similarly, if n = 3, then e +e +e = 2 and, up to isomorphism, there are 4 cases. It can
1 2 3
be easily checked that there is a packing in each of these cases (Figure 3).
x1 y1 x1 y1 x1 y1 x1 y1
x2 y2 x2 y2 x2 y2 x2 y2
x3 y3 x3 y3 x3 y3 x3 y3
Figure 3. Graphs with n = 3 vertices and 2 edges.
Now assume that n 4 and the theorem holds for all smaller values. If there is some
≥
v V with d (v) = n 1, then there are fewer than n/2 1 edges not adjacent to v. Let
i 3
∈ − −
u V be a vertex who has no neighbors in (V V ) v. If uv / E , then we pack G v
3−i 1 2 3 i
∈ ∪ − ∈ −
and G u by induction and extend it by assigning v to u. If uv E , then there is some
3−i 3
− ∈
vertex w V v with degree at least 1, otherwise we could easily send v to its non-neighbor
i
∈ −
and V v can be sent arbitrarily. So, with this w V v where d(w) 1, we assign w
1 i
− ∈ − ≥
to u and then pack G w and G u by induction. We can now assume that for all
i 3−i
− −
v V V , d (v) n 2 and proceed in three cases:
1 2 3
∈ ∪ ≤ −
Case 1: There exists a vertex x V such that d (x) = d (x) = 0. If there exists a
i i 3
∈
y V with d (y) + d (y) 2, then G x and G y pack by induction and this
3−i 3−i 3 i 3−i
∈ ≥ − −
packing can be extended to the original graphs by assigning x to y. So we may assume that
d (y)+d (y) 1 for all y V and we can assume there is at least one y V with
3−i 3 3−i 3−i
≤ ∈ ∈
d (y)+d (y) = 1 or else the graph triple packs trivially. Moreover, we can assume there is
3−i 3
at least one y V with d (y) = 1, say yz E or else all edges incident to V vertices
3−i 3−i 3−i 3−i
∈ ∈
are yellow and this too packs trivially since G would be a yellow matching with E = .
3 2
∅
Since at most n edges are accounted for with endpoints in V , then there is a w V with
3−i i
∈
d (w) + d (w) 2 and wz / E . Then, by induction, there is a packing G w,x and
i 3 3 i
≥ ∈ − { }
G y,z which can be extended to a packing of the original graphs by mapping w to z
3−i
−{ }
and x to y.
5
Case 2: There is some x V with d (x) = 0, but d (x) > 0. If d (x) 2, then we find
i i 3 3
∈ ≥
an allowed partner z V , pack G x and G z by induction, and extend the packing
3−i i 3−i
∈ − −
by assigning x to z. So, we may assume d (x) = 1. Let xy E be this edge. Since there
3 3
∈
are no isolated vertices in V (as otherwise we would be done by Case 1), let v V y
3−i 3−i
∈ −
such that d (v)+d (v) 1. Then, there is a packing of G x and G v by induction
3−i 3 i 3−i
≥ − −
which can be extended to the original graphs by mapping x to v.
Case 3: δ > 0 and δ > 0. Without loss of generality, assume that e e , so e < 3n/4.
1 2 1 2 1
≤
There are more than n/4 non-trivial tree components in G and, since δ 1, more than n/2
1 1
(cid:4) (cid:5) ≥
vertices of degree 1. Further, e 3n 2 e e < n/2, so there exists a vertex x V
3 ≤ 2 − − 1 − 2 ∈ 1
with d(x) = 1 so x only has a white neighbor. Let y G such that d (y) 1. Consider the
2 3
∈ ≥
graph obtained by removing x,y from V V and adding to E all edges from N(x) to
1 2 3
{ } ∪
N(y). This results in a net change of at least two fewer edges so that, by induction, there is
a packing of G x and G y which extends to a packing of the original graphs by mapping
1 2
x to y. (cid:3) − −
Lemma 7 along with the following corollary will serve as a base case for our proof of
Theorem 6.
Corollary 8. Suppose G = G = n 2 and G is a triple (G ,G ,G ). If e +e +e n,
1 2 1 2 3 1 2 3
| | | | ≥ ≤
then either:
(1) G has a packing, or
(2) For some i 1,2 , some v V is adjacent to all vertices in V , or
i 3−i
∈ { } ∈
(3) n = 2 and G = G = K .
1 ∼ 2 ∼ 2
Proof: If n = 2 and e +e +e = 2, then the result is clear. If n = 3 and G has no white
1 2 3 i
edges for some i 1,2 , then the problem is equivalent to finding a matching in the com-
∈ { }
plement of G and the result follows from Hall’s Theorem. Similarly, if there are no yellow
3
edges, then the result follows from Figure 1, so it must be the case that e = e = e = 1.
1 2 3
Up to isomorphism, there are only 3 cases and it is clear that in each case there is a packing
(Figure 4). For n 4, the result follows from Lemma 7. (cid:3)
≥
x1 y1 x1 y1 x1 y1
x2 y2 x2 y2 x2 y2
x3 y3 x3 y3 x3 y3
Figure 4. Graphs with n = 3 vertices and 3 edges.
Proof of Theorem 6
4.
Let G = (G ,G ,G ) of size n be a counterexample to Theorem 6 with the smallest size.
1 2 3
Notice that n 4, since otherwise Corollary 8 applies. Also, we assume E = or else
3
≥ (cid:54) ∅
Theorem 4 applies.
Lemma 9. ∆ n 2.
3
≤ −
6
Proof: Suppose that there exist v V and w V such that N (v) = V w. Let G(cid:48) be
1 2 3 2
∈ ∈ −
obtained from G by deleting all n 1 edges connecting v with V and all edges (maybe zero)
2
−
connecting w with V . Let A = N (v), B = N (w), a = A and b = B . If a = 0 or b = 0,
1 G(cid:48) G(cid:48)
| | | |
then if we pack G v with G w, placing v on w does not create conflicts. And G v w
1 2
− − − −
has at most (2n 3) (n 1) = n 2 edges. Such pairs always pack. So assume a 1 and
− − − − ≥
b 1.
≥
Let X and Y be the vertex sets of the component of G(cid:48) containing v and w, respectively.
Possibly, X = Y. Graph G(cid:48) X Y has 2n x y vertices and at most
− − − −
2n 3 (n 1) (x 1) (y 1) = n x y
− − − − − − − − −
edges. So, it has at least (2n x y) (n x y) = n components, and thus at least
− − − − −
x + y of them have no edges, i.e. are singletons. Either at least x of them are in V or
2
at least y of them are in V . Suppose the former holds (the proof of the other case is
1
symmetric). Then we place v on w, the white neighbors of v on singletons in V , and con-
2
sider the remaining subgraph G(cid:48)(cid:48) with parts G(cid:48)(cid:48) and G(cid:48)(cid:48). Any packing of G(cid:48)(cid:48) with G(cid:48)(cid:48) does
1 2 1 2
not create conflicts in our placement. Furthermore, G(cid:48)(cid:48) has 2n 2x vertices and at most
−
2n 3 (n 1) (x 1) b = n 1 x b edges. Again, the number of edges in G(cid:48)(cid:48) is
− − − − − − (cid:3) − − −
less than the sizes of parts.
Lemma 10. ∆ ,∆ n 3.
1 2
≤ −
Proof: Suppose v,v(cid:48) V and N (v) = V v v(cid:48).
1 1 1
∈ − −
Case 1: There is w V N(v) with no neighbors in V . Send v to w. Any packing of
2 2
∈ −
the resulting triple G(cid:48) = (G v,G w,G(cid:48)) extends to a packing of G. Since G(cid:48) has at
1 − 2 − 3
most 2n 3 (n 2) = n 1 edges, by Corollary 8, it packs unless it has a vertex of yellow
− − − −
degree n 1. But this is not the case by Lemma 9.
−
Case 2: Every w V N(v) has a white neighbor. Let W(cid:48) be the set of vertices in V
2 2
∈ −
reachable in G from V , and let W = V W(cid:48). Since G W has at least (n 2) + W(cid:48)
1 2
− − − | |
edges, W(cid:48) n 1. So W = and if the white degree of v(cid:48) is a, then
| | ≤ − (cid:54) ∅
(1) E(G[W]) (2n 3) (n 2) a W(cid:48) = W 1 a.
| | ≤ − − − − −| | | |− −
Let W be the vertex set of a smallest tree component in G[W], y be a vertex of degree 1 in
1
G[W ] and y(cid:48) be the white neighbor of y. Suppose the white degree of y(cid:48) is b. We send v to y,
1
v(cid:48) toy(cid:48) andadda(b 1)yellowedgesconnectingthewhiteneighborsofv(cid:48) withthe(necessarily
−
white)neighborsofy(cid:48) distinctfromy. IftheresultingtripleG(cid:48) = (G v v(cid:48),G w w(cid:48),G(cid:48))
1− − 2− − 3
packs, then because of the added edges, this extends to a packing of G. Suppose it does not.
Triple G(cid:48) has 2(n 2) vertices and at most
−
(2) 2n 3 (n 2) a b+a(b 1) = n 1 2a+b(a 1)
− − − − − − − − −
edges.
If a 1, then b(a 1) 0. Also, (2) is at most n 2, and by Corollary 8, either some
≤ − ≤ −
V has a vertex z with yellow degree n 2, or the new graphs are each K . However, if
i 2
−
each of the new graphs is K and v originally had white degree 2, then the case condition
2
implies that there are at least 6 > 2n 3 edges in the original graph. So we assume that
−
z V has yellow degree n 2 in G(cid:48). In this case, we need a = 0 and all edges of G apart
i
∈ −
from yy(cid:48) are incident either with v or with z and z / v,v(cid:48) . Thus, vertices in V y y(cid:48)
2
∈ { } − −
7
have no white neighbors, a contradiction to the case. Further, if b = 1, then there are at
most n 3 edges in the resulting graph so G(cid:48) packs by Corollary 8. So let a 2 and
− ≥
b 2. In particular, 2 W n. By (1), G[W] has at least a + 1 tree components,
≥ ≤ | | ≤
3 b+1 W W /(a+1) n/(a+1) and thus 2 b 1+n/(a+1). Since a 2,
1
≤ ≤ | | ≤ | | ≤ ≤ ≤ − ≥
then
(cid:18) (cid:19)
n
E(G(cid:48)) n 1 2a+ 1 (a 1)
| | ≤ − − a+1 − −
a n
= n 3a+n
− a+1 − a+1
a
n+n 3a 3
≤ a+1 − −
a
n+n 9 < 2(n 2) 3
≤ a+1 − − −
Since G(cid:48) does not pack, by induction (the last strict inequality ensures that examples from
Figure 1 do not appear) some vertex z in G(cid:48) has d (z) = n 2 or d (z) = n 3 for some
3 i
− −
i = 1 or 2. But d (z) = n 3, since we deleted at least n 2 + a + b n + 1 edges from
i
(cid:54) − − ≥
2n 3 in G and have not added white edges. Similarly, since we have not added any yellow
−
edges incident to V W , we cannot have d (z) = n 2 if z V W .
2 1 3 2 1
− − ∈ −
The casewhenz V and hasyellowdegreen 2 isalso forbiddensince forthis tohappen,
1
∈ −
every vertex in V must be incident to a yellow edge. However, G[W] has at least a+1 3
2
≥
components, of which only W is incident to yellow edges. Finally, we cannot have z W ,
1 1
∈
or else we must have created a star by adding yellow edges from z W to all vertices in
1
∈
V v v(cid:48). But this implies a packing by Theorem 4 since if a = n 2, then e = 2n 4
1 1
and−e −= 1 and G contained no yellow edges. (cid:3) − −
2
Lemma 11. Every vertex of G has a white neighbor.
Proof: Suppose v V has no white neighbor.
∈
Case 1: v is isolated in G. Without loss of generality, assume v V . If any w V has
1 2
∈ ∈
degree at least 2 in G then placing v on w decreases e +e +e by at least 2. If the new
1 2 3
triple packs, then this extends to G, otherwise by Lemmas 9 and 10, it is one of the examples
from Figure 1. Suppose that (G v,G w) is an example from Figure 1, then there must
1 2
− −
be exactly 2(n 1) 3 edges and so d(w) = 2. If both edges adjacent to w were white, then
− −
G originally had no yellow edges and the theorem holds by Theorem 4. If both edges were
yellow, since every graph in the Figure 1 contains 3 vertices of positive degree, there is some
v(cid:48) V N(w) with degree at least 1. Then (G v(cid:48),G w) contains strictly fewer than
1 1 2
∈ − − −
2(n 1) 3 edges and therefore, by Theorem 4, pack. This packing can be extended to a
− −
packing on G. Finally assume that w has exactly one neighbor w(cid:48) V and one neighbor in
2
∈
V . Again, we can choose some v(cid:48) V N(w) with positive degree. Create a new graph
1 1
∈ −
triple by removing v(cid:48) and w and adding yellow edges from w(cid:48) to N(v(cid:48)). This new triple has
exactly 2(n 1) 3 edges. By Lemmas 9 and 10 (and since it has at least one yellow edge),
− −
the new triple packs and can be extended to a packing of G.
So suppose the total degree of each w V is at most 1. If at least one w V is isolated,
2 2
∈ ∈
then symmetrically, each v(cid:48) V has degree at most 1 and G packs by Corollary 8. Thus
1
∈
8
d(w) = 1 for each w V . If there are no yellow edges, then we are done by Theorem 4. So
2
∈
let wv(cid:48) E . If all vertices in V v(cid:48) are isolated, then the total degree sum of G is at most
3 1
∈ −
n+∆ (G) 2n 3 and so e +e +e < n, a contradiction to Corollary 8. Otherwise, let
3 1 2 3
≤ −
u V v(cid:48) be a vertex of maximum degree and send u to w. If (G u,G w) packs, then
1 1 2
∈ − − −
thispackingextendstoapackingofG. Ifitdoesnotpack, thenbyinduction, (G u,G w)
1 2
− −
is an example from Figure 1 and d(u) = 1. However, each example in Figure 1 contains a
graph with multiple vertices of degree at least 2, contradicting the maximality of u.
Case 2: N (v) = . Let v V such that N (v) = and suppose w(cid:48) N (v). Among
3−i 1 1 2
(cid:54) ∅ ∈ ∅ ∈
the vertices in V N (v) with maximal degree, let w be a vertex that minimizes d (w). We
2 2 3
−
sendv tow andconsiderthetripleformedbyremovingthesetwovertices. Ifd(v)+d(w) > 2,
thentheremaininggraphtriplepacksbyinductionandthepackingextendstoapackingofG.
Therefore, by Case 1, we may assume that d(v) = d(w) = 1. By induction, (G v,G w)
1 2
− −
must be an example from Figure 1, as otherwise the graphs pack.
However, by the maximality of d(w), all vertices in V w(cid:48) must have degree at most 1 in
2
−
G and, hence, in G w. By inspection, G w is either G(1) or G(3) in Figure 1, as all
2 2
− −
other graphs have multiple vertices with degree at least 2. Since H(1) and H(2) each have an
isolated vertex that, by Case 1, was not isolated in G , we must have removed an incident
1
yellow edge when deleting w. Both G(1) and G(3) have at least 4 vertices adjacent to at
exactly one white edge. In the process of removing v and w from G, we have removed at
most 2 edges incident to V . Thus, in G , there must also have been a vertex with degree 1
2 2
adjacent to a white edge, contradicting our choice of w. (cid:3)
Proof of Theorem 6: Let G be our minimum counterexample. If G has no yellow edges,
then the original Theorem 4 applies. So suppose G has a yellow edge xy with x V and
1
∈
y V . Since E(G) 2n 3 < 2n, there are vertices of degree at most 1. We may assume
2
∈ | | ≤ −
that v V and d(v) 1. By Lemma 11, v has a white neighbor, v(cid:48) (possibly v(cid:48) = x). We
1
∈ ≤
send v to y and add yellow edges from v(cid:48) to each white neighbor of y. Then we obtain the
triple G(cid:48) with exactly two edges less. Since we have at least one yellow edge (connecting v(cid:48)
with a white neighbor of y), we do not get a graph from Figure 1. So by Lemmas 9 and 10,
(cid:3)
the theorem is proved.
Acknowledgement: We would like to thank Gexin Yu for helpful comments.
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Alfre´d Re´nyi Institute of Mathematics, Budapest, Hungary, and, Department of Math-
ematics, Central European University, Budapest, Hungary
E-mail address: [email protected]
Department of Mathematics, University of Illinois, Urbana, IL 61801, USA, and, Sobolev
Institute of Mathematics, Novosibirsk, Russia
E-mail address: [email protected]
Department of Mathematics, University of Illinois, Urbana, IL 61801, USA
E-mail address, Corresponding author: [email protected]
Department of Mathematics, University of Illinois, Urbana, IL 61801, USA
E-mail address: [email protected]
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