Let \(G\) be a connected graph of order \(n\), and suppose that \(n = \sum_{i=1}^{k}n_i\), where \(n_1, n_2, \ldots,n_n\) are integers with at least two. A spanning subgraph is called a path-factor if each component of it is a path of order at least two. In [Y. Chen, F. Tian, B, Wei, Degree sums and path-factors in graphs, Graphs and Combin. \(17 (2001),61-71.]\), Chen et al. gave a degree sum condition for the existence of a path-factor consisting of paths of order \(n_1, n_2, \ldots, n_k\). In this paper, for 2-connected graphs, we generalize this result.

Let \(G\) be a graph with \(n\) vertices and \(\mu_1, \mu_2, \ldots, \mu_n\) be the Laplacian eigenvalues of \(G\). The Laplacian-energy-like graph invariant \(\text{LEL}(G) = \sum_{i=1}^{n} \sqrt{\mu_i}\) has been defined and investigated in [1]. Two non-isomorphic graphs \(G_1\) and \(G_2\) of the same order are said to be \(\text{LEL}\)-equienergetic if \(\text{LEL}(G_1) = \text{LEL}(G_2)\). In [2], three pairs of \(\text{LEL}\)-equienergetic non-cospectral connected graphs are given. It is also claimed that the \(\text{LEL}\)-equienergetic non-cospectral connected graphs are relatively rare. It is natural to consider the question: Whether the number of the \(\text{LEL}\)-equienergetic non-cospectral connected graphs is finite? The answer is negative, because we shall construct a pair of \(\text{LEL}\)-equienergetic non-cospectral connected graphs of order \(n\), for all \(n \geq 12\) in this paper.

The status of a vertex \(v\) in a graph is the sum of the distances between \(v\) and all vertices. The status sequence of a graph is the list of the statuses of all vertices arranged in nondecreasing order. It is well known that non-isomorphic graphs may have the same status sequence. This paper gives a sufficient condition for a graph \(G\) with the property that there exists another graph \(G’\) such that \(G’\) and \(G\) have the same status sequence and \(G’\) is not isomorphic to \(G\).

We give combinatorial proofs of some binomial and $q$-binomial identities in the literature, such as

\[\sum\limits_{k={-\infty}}^{\infty}(-1)^kq^{\frac{(9k^2+3k)}{2}}\binom{2n}{n+3k}=(1+q^n)\prod\limits_{k=1}^{n-1}(1+q^k+q^{2k})(n\geq 1)\]

and

\[\sum\limits_{k=0}^{\infty} \binom{3n}{2k}(-3)^k=(-8)^n.\]

Two related conjectures are proposed at the end of this paper.

In the spirit of Ryser’s theorem, we prove sufficient conditions on \(k\), \(\ell\), and \(m\) so that \(k \times \ell \times m\) Latin boxes, i.e., partial Latin cubes whose filled cells form a \(k \times \ell \times m\) rectangular box, can be extended to a \(k \times n \times m\) Latin box, and also to a \(k \times n \times m\) Latin box, where \(n\) is the number of symbols used, and likewise the order of the Latin cube. We also prove a partial Evans-type result for Latin cubes, namely that any partial Latin cube of order \(n\) with at most \(n-1\) filled cells is completable, given certain conditions on the spatial distribution of the filled cells.

A star-factor of a graph \(G\) is a spanning subgraph of \(G\) such that each component is a star. An edge-weighting of \(G\) is a function \(w: E(G) \rightarrow \mathbb{N}^+\), where \(\mathbb{N}^+\) is the set of positive integers. Let \(\Omega\) be the family of all graphs \(G\) such that every star-factor of \(G\) has the same weight under some fixed edge-weighting \(w\). The open problem of characterizing the class \(\Omega\), posed by Hartnell and Rall, is motivated by the minimum cost spanning tree and the optimal assignment problems. In this paper, we present a simple structural characterization of the graphs in \(\Omega\) that have girth at least five.

We show that whenever the length four words over a three letter alphabet are two-colored, there must exist a monochromatic combinatorial line. We also provide some computer generated lower bounds for some other Hales-Jewett numbers.

This paper introduces a method for finding closed forms for certain sums involving squares of binomial coefficients. We use this method to present an alternative approach to a problem of evaluating a different type of sums containing squares of the numbers from
Catalan’s triangle.

In this paper, we deal with a special kind of hypergraph decomposition. We show that there exists a decomposition of the 3-uniform hypergraph \(\lambda K_v^{(3)}\) into a special kind of hypergraph \(K_{4}^{(3)} – e\) whose leave has at most two edges, for any positive integers \(v \geq 4 \) and \(\lambda\).

For a connected graph \(G\) of order \(n \geq 2\) and a linear ordering \(s = v_1, v_2, \ldots, v_n\) of \(V(G)\), define \(d(s) = \sum_{i=1}^{n-1} d(v_i, v_{i+1})\), where \(d(v_i, v_{i+1})\) is the distance between \(v_i\) and \(v_{i+1}\). The traceable number \(t(G)\) and upper traceable number \(t^+(G)\) of \(G\) are defined by \(t(G) = \min\{d(s)\}\) and \(t^+(G) = \max\{d(s)\}\), respectively, where the minimum and maximum are taken over all linear orderings \(s\) of \(V(G)\). The traceable number \(t(v)\) of a vertex \(v\) in \(G\) is defined by \(t(v) = \min\{d(s)\}\), where the minimum is taken over all linear orderings \(s\) of \(V(G)\) whose first term is \(v\). The \({maximum\; traceable \;number}\) \(t^*(G)\) of \(G\) is then defined by \(t^*(G) = \max\{t(v) : v \in V(G)\}\). Therefore, \(t(G) \leq t^*(G) \leq t^+(G)\) for every nontrivial connected graph \(G\). We show that \(t^*(G) \leq \lfloor \frac{t(G)+t^+(G)+1}{2}\rfloor\) for every nontrivial connected graph \(G\) and that this bound is sharp. Furthermore, it is shown that for positive integers \(a\) and \(b\), there exists a nontrivial connected graph \(G\) with \(t(G) = a\) and \(t^*(G) = b\) if and only if \(a \leq b \leq \left\lfloor \frac{3n}{2} \right\rfloor\).