We give a necessary and sufficient condition of Hall’s type for a family of sets of even cardinality to be decomposable into two subfamilies having a common system of distinct representatives. An application of this result to partitions of Steiner Triple Systems into small configurations is presented.

In this paper, we construct \(2\)-factorizations of \(K_n\) (\(n\) odd) containing a specified number, \(k\), of \(6\)-cycles, for all integers \(k\) between 0 and the maximum possible expected number of \(6\)-cycles in any \(2\)-factorization, and for all odd \(n\), with no exceptions.

We deal with \((a,d)\)-face antimagic labelings of a certain class of plane quartic graphs. A connected plane graph \(G = (V, E, F)\) is said to be \((a,d)\)-\({face\; antimagic}\) if there exist positive integers \(a\) and \(d\), and a bijection \(g : E(G) \rightarrow \{1,2,…,|E(G)|\}\) such that the induced mapping \(\varphi_g : F(G) \rightarrow {N}\), defined by \(\varphi_g(f) = \sum\{g(e): e \in E(G) \text{ adjacent to face } f\}\), is injective and \(\varphi_g(F) = \{a,a+d,…,a+ (|F(G)| – 1)d\}\).

Let \(G\) be a graph with vertex set \(V\) and edge set \(E\). A vertex labelling \(f : V \rightarrow \{0,1\}\) induces an edge labelling \(\overline{f} : E \rightarrow \{0,1\}\) defined by \(\overline{f}(uv) = |f(u) – f(v)|\). Let \(v_f(0), v_f(1)\) denote the number of vertices \(v\) with \(f(v) = 0\) and \(f(v) = 1\) respectively. Let \(e_f(0), e_f(1)\) be similarly defined. A graph is said to be cordial if there exists a vertex labeling \(f\) such that \(|v_f(0) – v_f(1)| \leq 1\) and \(|e_f(0) – e_f(1)| \leq 1\). In this paper, we show that for every positive integer \(t\) and \(n\) the following families are cordial: (1) Helms \(H_{n}\). (2) Flower graphs \(FL_{n}\). (3) Gear graphs \(G_{n}\). (4) Sunflower graphs \(SFL_{n}\). (5) Closed helms \(CH_{n}\). (6) Generalised closed helms \(CH(t,n)\). (7) Generalised webs \(W(t, n)\).

A cycle \(C\) of a graph \(G\) is called a \(q\)-dominating cycle if every vertex of \(G\) which is not contained in \(C\) is adjacent to at least \(q\) vertices of \(C\). Let \(G\) be a \(k\)-connected graph with \(k \geq 2\). We present a sufficient condition, in terms of the degree sum of \(k + 1\) independent vertices, for \(G\) to have a \(qg\)-dominating cycle. This is an extension of a 1987 result by J.A. Bondy and G. Fan. Furthermore, examples will show that the given condition is best possible.

In an earlier paper [11], we proved that there does not exist any \(\Delta\)-critical graph of even order with five major vertices. In this paper, we prove that if \(G\) is a \(\Delta\)-critical graph of odd order \(2n+1\) with five major vertices, then \(e(G) = n\Delta+1\). This extends an earlier result of Chetwynd and Hilton, and also completes our characterization of graphs with five major vertices. In [9], we shall apply this result to establish some results on class 2 graphs whose core has maximum degree two.

In this paper, uniquely list colorable graphs are studied. A graph \(G\) is said to be uniquely \(k\)-list colorable if it admits a \(k\)-list assignment from which \(G\) has a unique list coloring. The minimum \(k\) for which \(G\) is not uniquely \(k\)-list colorable is called the \(m\)-number of \(G\). We show that every triangle-free uniquely colorable graph with chromatic number \(k+1\) is uniquely \(k\)-list colorable. A bound for the \(m\)-number of graphs is given, and using this bound it is shown that every planar graph has \(m\)-number at most \(4\). Also, we introduce list criticality in graphs and characterize all \(3\)-list critical graphs. It is conjectured that every \(\chi_\ell’\)-critical graph is \(\chi’\)-critical, and the equivalence of this conjecture to the well-known list coloring conjecture is shown.

A labeling \(f\) of the vertices of a graph \(G\) is said \(k\)-\({equitable}\) if each weight induced by \(f\) on the edges of \(G\) appears exactly \(k\) times. A graph \(G\) is said \({equitable}\) if for every proper divisor \(k\) of its size, the graph \(G\) has a \(k\)-equitable labeling.

A graph \(G\) is a corona graph if \(G\) is obtained from two graphs, \(G_1\) and \(G_2\), taking one copy of \(G_ 1\), which is supposed to have order \(p\), and \(p\) copies of \(G_2\), and then joining by an edge the \(k^{th}\) vertex of \(G_1\) to every vertex in the \(k^{th}\) copy of \(G_2\). We denote \(G\) by \(G_1 \otimes G_2\).

In this paper, we proved that the corona graph \(C_n \otimes K_1\) is equitable. Moreover, we show \(k\)-equitable labelings of the corona graph \(C_m \otimes nK_1\), for some values of the parameters \(k, m,\) and \(n\).

In this paper, we derive a necessary existence condition involving the parameters of a balanced array (B-array) with two symbols and of strength \(t = 8\). Consequently, we demonstrate that the existence condition derived here can provide us with useful information on the maximum number of constraints for B-arrays with a given number of columns.