General results on the enumeration of strings in Dyck paths.
For each vertex s of the vertex subset S of a simple graph G, we define Boolean variables p = p(s,S), q = q(s,S) and r = r(s,S) which measure existence of three kinds of S-private neighbours (S-pns) of s. A 3-variable Boolean function f = f(p,q,r) may be considered as a compound existence property of S-pns. The subset S is called an f-set of G if f = 1 for all s ∈ S and the class of f-sets of G is denoted by . Only 64 Boolean functions f can produce different classes , special cases of which include...
There are four resolvable Steiner triple systems on fifteen elements. Some generalizations of these systems are presented here.
The Gyárfás tree packing conjecture asserts that any set of trees with 2,3,...,k vertices has an (edge-disjoint) packing into the complete graph on k vertices. Gyárfás and Lehel proved that the conjecture holds in some special cases. We address the problem of packing trees into k-chromatic graphs. In particular, we prove that if all but three of the trees are stars then they have a packing into any k-chromatic graph. We also consider several other generalizations of the conjecture.
The generalized -connectivity of a graph was introduced by Chartrand et al. in 1984. As a natural counterpart of this concept, Li et al. in 2011 introduced the concept of generalized -edge-connectivity which is defined as and , where denotes the maximum number of pairwise edge-disjoint trees in such that for . In this paper we prove that for any two connected graphs and we have , where is the Cartesian product of and . Moreover, the bound is sharp. We also obtain the...