In this paper, we investigate the existence of nontrivial solutions for the equation \(y(G \Box H) – \gamma(G) \gamma(H)\) fixing one factor. For the complete bipartite graphs \(K_{m,n}\), we characterize all nontrivial solutions when \(m = 2, n \geq 3\) and prove the nonexistence of solutions when \(m \geq 2, n \leq 3\). In addition, it is proved that the above equation has no nontrivial solution if \(A\) is one of the graphs obtained from \(G\), the cycle of length \(n\), either by adding a vertex and one pendant edge joining this vertex to any vertex to any \(v\in V(C_n)\), or by adding one chord joining two alternating vertices of \(C_n\).

For a graph \(G = (V(G), E(G))\), let \(i(G)\) be the number of isolated vertices in \(G\). The isolated toughness of \(G\) is defined as
\(I(G) = \min\left\{\frac{|S|}{i(G-S)}: S \subseteq V(G), i(G-S) \geq 2\right\}\) if \(G\) is not complete; \(I(G) = |V(G)|-1\) otherwise. In this paper, several sufficient conditions in terms of isolated toughness are obtained for the existence of \([a, b]\)-factors avoiding given subgraphs, e.g., a set of vertices, a set of edges and a matching, respectively.

In a graph \(G\), the distance \(d(u,v)\) between a pair of vertices \(u\) and \(v\) is the length of a shortest path joining them. The eccentricity \(e(u)\) of a vertex \(u\) is the distance to a vertex farthest from \(u\). The minimum eccentricity is called the radius of the graph and the maximum eccentricity is called the diameter of the graph. The radial graph \(R(G)\) based on \(G\) has the vertex set as in \(G\). Two vertices \(u\) and \(v\) are adjacent in \(R(G)\) if the distance between them in \(G\) is equal to the radius of \(G\). If \(G\) is disconnected, then two vertices are adjacent in \(R(G)\) if they belong to different components. The main objective of this paper is to find a necessary and sufficient condition for a graph to be a radial graph.

Let \(\{T, T’\}\) be a Latin bitrade. Then \(T\) (and \(T’\)) is said to be \((r,c,e)\)-homogeneous if each row contains precisely \(r\) entries, each column contains precisely \(c\) entries, and each entry occurs precisely \(e\) times. An \((r,c,e)\)-homogeneous Latin bitrade can be embedded on the torus only for three parameter sets, namely \((r,c,e) = (3,3,3), (4,4,2)\), or \((6,3,2)\). The first case has been completely classified by a number of authors. We present classifications for the other two cases.

In this paper, we prove an interesting property of rook polynomials for \(2\)-D square boards and extend that for rook polynomials for \(3\)-D cubic, and \(r\)-D “hypercubic” boards. In particular, we prove that for \(r\)-D rook polynomials the modulus of the sum of their roots equals their degree. We end with some further questions, mainly for the \(2\)-D and \(3\)-D case, that could serve as future projects.

Let \(G\) be a finite graph and \(H\) be a subgraph of \(G\). If \(V(H) = V(G)\), then the subgraph \(H\) is called a \({spanning \;subgraph}\) of \(G\). A spanning subgraph \(H\) of \(G\) is called an \({F-factor}\) if each component of \(H\) is isomorphic to \(F\). Further, if there exists a subgraph of \(G\) whose vertex set is \(\lambda V(G)\) and can be partitioned into \(F\)-factors, then it is called a \({\lambda-fold \;F-factor}\) of \(G\), denoted by \(S_\lambda(1,F,G)\). A \({large \; set}\) of \(\lambda\)-fold \(F\)-factors in \(G\) is a partition \(\{\mathcal{B}_i\}_{i}\) of all subgraphs of \(G\) isomorphic to \(F\), such that each \((X, \mathcal{B}_i)\) forms a \(\lambda\)-fold \(F\)-factor of \(G\). In this paper, we investigate the large set of \(\lambda\)-fold \(P_3\)-factors in \(K_{v,v}\) and obtain its existence spectrum.

Let \(k \geq 1\), \(l \geq 3\), and \(s \geq 5\) be integers. In \(1990\), Erdős and Faudree conjectured that if \(G\) is a graph of order \(4k\) with \(\delta(G) \geq 2k\), then \(G\) contains \(k\) vertex-disjoint \(4\)-cycles. In this paper, we consider an analogous question for \(5\)-cycles; that is to say, if \(G\) is a graph of order \(5k\) with \(\delta(G) \geq 3k\), then \(G\) contains \(k\) vertex-disjoint \(5\)-cycles? In support of this question, we prove that if \(G\) is a graph of order \(5k\) with \(\omega_2(G) \geq 6l – 2\), then, unless \(\overline{K_{l-2}} + K_{2l+1,2l+1} \subseteq G \subseteq K_{l-2} + K_{2l+1,2l+1}\), \(G\) contains \(l – 1\) vertex-disjoint \(5\)-cycles and a path of order \(5\), which is vertex-disjoint from the \(l – 1\) \(5\)-cycles. In fact, we prove a more general result that if \(G\) is a graph of order \(5k + 2s\) with \(\omega_2(G) \geq 6k + 2s\), then, unless \(\overline{K_{k}} + K_{2k+s,2k+s} \subseteq G \subseteq K_{k} + K_{2k+s,2k+s}\), \(G\) contains \(k+1\) vertex-disjoint \(5\)-cycles and a path of order \(2s – 5\), which is vertex-disjoint from the \(k + 1\) \(5\)-cycles. As an application of this theorem, we give a short proof for determining the exact value of \(\text{ex}(n,(k + 1)C_5)\), and characterize the extremal graph.

In this paper, we present the complex factorizations of the Jacobsthal and Jacobsthal Lucas numbers by determinants of tridiagonal matrices.

In this paper, we find families of \((0, -1, 1)\)-tridiagonal matrices whose determinants and permanents equal the negatively subscripted Fibonacci and Lucas numbers. Also, we give complex factorizations of these numbers by the first and second kinds of Chebyshev polynomials.

We classify all finite near hexagons which satisfy the following properties for a certain \(t_2 \in \{1,2,4\}\):(i) every line is incident with precisely three points;(ii) for every point \(x\), there exists a point \(y\) at distance \(3\) from \(x\);(iii) every two points at distance \(2\) from each other have either \(1\) or \(t_2 + 1\) common neighbours;(iv) every quad is big. As a corollary, we obtain a classification of all finite near hexagons satisfying (i), (ii) and (iii) with \(t_2\) equal to \(4\).