In [Kit1] Kitaev discussed simultaneous avoidance of two \(3\)-patterns with no internal dashes, that is, where the patterns correspond to contiguous subwords in a permutation. In three essentially different cases, the numbers of such \(n\)-permutations are \(2^{n-1}\), the number of involutions in \(S_n\), and \(2^{E_n}\), where \(E_n\) is the \(n\)-th Euler number. In this paper we give recurrence relations for the remaining three essentially different cases.

To complete the descriptions in [Kit3] and [KitMans], we consider avoidance of a pattern of the form \(x-y-z\) (a classical \(3\)-pattern) and beginning or ending with an increasing or decreasing pattern. Moreover, we generalize this problem: we demand that a permutation must avoid a \(3\)-pattern, begin with a certain pattern, and end with a certain pattern simultaneously. We find the number of such permutations in case of avoiding an arbitrary generalized \(3\)-pattern and beginning and ending with increasing or decreasing patterns.

A graph \(G\) is called integral or Laplacian integral if all the eigenvalues of the adjacency matrix \(A(G)\) or the Laplacian matrix \(Lap(G) = D(G) – A(G)\) of \(G\) are integers, where \(D(G)\) denotes the diagonal matrix of the vertex degrees of \(G\). Let \(K_{n,n+1} \equiv K_{n+1,n}\) and \(K_{1,p}[(p-1)K_p]\) denote the \((n+1)\)-regular graph with \(4n+2\) vertices and the \(p\)-regular graph with \(p^2 + 1\) vertices, respectively. In this paper, we shall give the spectra and characteristic polynomials of \(K_{n,n+1} \equiv K_{n+1,n}\) and \(K_{1,p}[(p-1)K_p]\) from the theory on matrices. We derive the characteristic polynomials for their complement graphs, their line graphs, the complement graphs of their line graphs, and the line graphs of their complement graphs. We also obtain the numbers of spanning trees for such graphs. When \(p = n^2 + n + 1\), these graphs are not only integral but also Laplacian integral. The discovery of these integral graphs is a new contribution to the search of integral graphs.

Balakrishnan et al. \([1, 2]\) have shown that every graph is a subgraph of a graceful graph and an elegant graph. Also Liu and Zhang \([4]\) have shown that every graph is a subgraph of a harmonious graph. In this paper we prove a generalization of these two results that any given set of graphs \(G_1,G_1,\ldots,G_i\) can be packed into a graceful/harmonious/elegant graph.

We consider compositions or ordered partitions of the natural number n for which the largest (resp. smallest) summand occurs in the first position of the composition.

Let \(m \geq 4\) be a positive integer and let \({Z}_m\) denote the cyclic group of residues modulo \(m\). For a system \(L\) of inequalities in \(m\) variables, let \(R(L;2)\) (\(R(L;{Z}_m)\)) denote the minimum integer \(N\) such that every function \(\Delta: \{1,2,\ldots,N\} \to \{0,1\}\) (\(A: \{1,2,\ldots,N\} \to {Z}_m\)) admits a solution of \(L\), say \((z_1,\ldots,z_m)\), such that \(\Delta(x_1) = \Delta(x_2) = \cdots = \Delta(x_m)\) (such that \(\sum_{i=1}^{m}\Delta(x_i) = 0\)). Define the system \(L_1(m)\) to consist of the inequality \(x_2 – x_1 \leq x_m – x_3\), and the system \(L_2(m)\) to consist of the inequality \(x_{m – 2}-x_{1} \leq x_m – x_{m-1}\); where \(x_1 < x_2 < \cdots < x_m\) in both \(L_1(m)\) and \(L_2(m)\). The main result of this paper is that \(R(L_1(m);2) = R(L_1(m);{Z}_m) = 2m\), and \(R(L_2(m);2) = 6m – 15\). Furthermore, we support the conjecture that \(R(L_1(m);2) = R(L_1(m);{Z}_m)\) by proving it for \(m = 5\).

In a given graph \(G\), a set \(S\) of vertices with an assignment of colors is a defining set of the vertex coloring of \(G\), if there exists a unique extension of the colors of \(S\) to a \(\chi(G)\)-coloring of the vertices of \(G\). A defining set with minimum cardinality is called a smallest defining set (of vertex coloring) and its cardinality, the defining number, is denoted by \(d(G, \chi)\). We study the defining number of regular graphs. Let \(d(n,r, \chi = k)\) be the smallest defining number of all \(r\)-regular \(k\)-chromatic graphs with \(n\) vertices, and \(f(n,k) = \frac{k-2}{2(k-1)} +\frac{2+(k-2)(k-3)}{2(k-1)}\). Mahmoodian and Mendelsohn (1999) determined the value of \(d(n,k, \chi = k)\) for all \(k \leq 5\), except for the case of \((n,k) = (10,5)\). They showed that \(d(n,k, \chi = k) = \lceil f(n,k) \rceil\), for \(k \leq 5\). They raised the following question: Is it true that for every \(k\), there exists \(n_0(k)\) such that for all \(n \geq n_0(k)\), we have \(d(n,k, \chi = k) = \lceil f(n,k) \rceil\)?

Here we determine the value of \(d(n,k, \chi = k)\) for each \(k\) in some congruence classes of \(n\). We show that the answer for the question above, in general, is negative. Also, for \(k = 6\) and \(k = 7\) the value of \(d(n,k, \chi = k)\) is determined except for one single case, and it is shown that \(d(10,5, \chi = 5) = 6\).

Let \((T_i)_{i\geq 0}\) be a sequence of trees such that \(T_{i+1}\) arises by deleting the \(b_i\) vertices of degree \(\leq 1\) from \(T_i\). We determine those trees of given degree sequence or maximum degree for which the sequence \(b_0, b_1, \ldots\) is maximum or minimum with respect to the dominance order. As a consequence, we also determine trees of given degree sequence or maximum degree that are of maximum or minimum Balaban index.