Let \(G = (V, E)\) be a graph with no isolated vertex. A classical observation in domination theory is that if \(D\) is a minimum dominating set of \(G\), then \(V \setminus D\) is also a dominating set of \(G\). A set \(D’\) is an inverse dominating set of \(G\) if \(D’\) is a dominating set of \(G\) and \(D’ \subseteq V \setminus D\) for some minimum dominating set \(D\) of \(G\). The inverse domination number of \(G\) is the minimum cardinality among all inverse dominating sets of \(G\). The independence number of \(G\) is the maximum cardinality of an independent set of vertices in \(G\). Domke, Dunbar, and Markus (Ars Combin. \(72 (2004), 149-160)\) conjectured that the inverse domination number of \(G\) is at most the independence number of \(G\). We prove this conjecture for special families of graphs, including claw-free graphs, bipartite graphs, split graphs, very well covered graphs, chordal graphs, and cactus graphs.

A \(\lambda\)-design on \(v\) points is a set of \(v\) distinct subsets (blocks) of a \(v\)-set such that any two different blocks meet in exactly \(\lambda\) points and not all of the blocks have the same size. Ryser’s and Woodall’s \(\lambda\)-design conjecture states that every \(\lambda\)-design can be obtained from a symmetric design by a certain complementation procedure. A result of Ryser and Woodall establishes that there exist two integers, \(r\) and \(r^*\), such that each point in a \(\lambda\)-design is in exactly \(r\) or \(r^*\) blocks. The main result of the present paper is that the \(\lambda\)-design conjecture is true for \(\lambda\)-designs with \(\gcd(r-1,r^*-1)=7\).

Improving on Domokos’s improvement of Swan’s theorem, we show that under certain conditions on a finite digraph, whenever \(p,q\) are vertices, then the number of even Eulerian paths from \(p\) to \(q\) is the same as the number of odd ones from \(p\) to \(q\).

Double-loop networks have been widely studied as architecture for local area networks. A double-loop network \(G(N;s_1,s_2)\) is a digraph with \(N\) vertices \(0,1,\ldots,N-1\) and \(2N\) edges of two types:

\(s_1-edge\): \(i \rightarrow i+s_1 \pmod{N}\); \(i=0,1,\ldots,N-1\).

\(s_2-edge\): \(i \rightarrow i+s_2 \pmod{N}\); \(i=0,1,\ldots,N-1\).

for some fixed steps \(1 \leq s_1 < s_2 < N\) with \(\gcd(N,s_1,s_2) = 1\). Let \(D(N;s_1,s_2)\) be the diameter of \(G\) and let us define \(D(N) = \min\{D(N;s_1,s_2) | 1 \leq s_1 < s_2 < N \text{ and } gcd(N,s_1,s_2) = 1\}\), and \(D_1(N) = \min\{D(N;1,s) | 1 < s < N\}\). If \(N\) is a positive integer and \(D(N) < D_1(N)\), then \(N\) is called a non-unit step integer or a nus integer. Xu and Aguild et al. gave some infinite families of 0-tight nus integers with \(D_1(N) – D(N) \geq 1\). In this work, we give a method for finding infinite families of nus integers. As application examples, we give one infinite family of 0-tight nus integers with \(D_1(N) – D(N) \geq 5\), one infinite family of 2-tight nus integers with \(D_1(N) – D(N) \geq 1\) and one infinite family of 3-tight nus integers with \(D_1(N) – D(N) \geq 1\).

Ramanujan’s \(\mathop{_1\psi_1}\)-summation formula is one of the fundamental identities in basic hypergeometric series. We review proofs of this identity and clarify its connections with other basic hypergeometric series transformations and formulae. In particular, we shall put our main emphasis on methods that can be used not only to provide deeper insight into Ramanujan’s \(\mathop{_1\psi_1}\)-summation formula, but also to derive new transformations and identities for basic hypergeometric series.

A diagonalised lattice is a two dimensional grid, where we add exactly one arbitrary diagonal in each square, and color each vertex black or white.We show that for every diagonalised lattice there is a walk from the left to the right, using only black vertices, if and only if there is no walk from the top to the bottom, using only white vertices.

A terrace for \(\mathbb{Z}_m\) is an arrangement \((a_1, a_2, \ldots, a_m)\) of the \(m\) elements of \(\mathbb{Z}_m\) such that the sets of differences \(a_{i+1} – a_i\) and \(a_i – a_{i+1}\) (\(i = 1, 2, \ldots, m-1\)) between them contain each element of \(\mathbb{Z}_m \setminus \{0\}\) exactly twice. For \(m\) odd, many procedures are available for constructing power-sequence terraces for \(\mathbb{Z}_m\); each such terrace may be partitioned into segments one of which contains merely the zero element of \(\mathbb{Z}_m\) whereas each other segment is either (a) a sequence of successive powers of a non-zero element of \(\mathbb{Z}_m\) or (b) such a sequence multiplied throughout by a constant. For \(n\) an odd prime power satisfying \(n \equiv 1\) or \(3 \pmod{8}\), this idea has previously been extended by using power-sequences in \(\mathbb{Z}_n\) to produce some \(\mathbb{Z}_m\) terraces \((a_1, a_2, \ldots, a_m)\) where \(m = n+1 = 2^\mu\), with \(a_{i+1} – a_i = -(a_{i+1+\mu} – a_{i+\mu})\) for all \(i \in [1, \mu-1]\). Each of these “da capo directed terraces” consists of a sequence of segments, one containing just the element \(0\) and another just containing the element \(n\), the remaining segments each being of type (a) or (b) above with each of its distinct entries \(z\) from \(\mathbb{Z}_n \setminus \{0\}\) evaluated so that \(1 \leq x \leq n-1\). Now, for many odd prime powers \(n\) satisfying \(n \equiv 1 \pmod{4}\), we similarly produce narcissistic terraces for \(\mathbb{Z}_{n+1}\); these have \(a_{i+1} – a_i = a_{m-i+1} – a_{m-i}\) for all \(i \in [1, \mu-1]\).

We determine the full Sylow \(p\)-subgroup of the automorphism group of transitive \(k\)-ary relational structures of order \(p^2\), \(p\) a prime. We then find the full automorphism group of transitive ternary relational structures of order \(p^2\), for those values of \(p\) for which \({A_p}\) is the only doubly-transitive nonabelian simple group of degree \(p\). Finally, we determine optimal necessary and sufficient conditions for two Cayley \(k\)-ary relational structures of order \(p^2\), \(k < p\), to be isomorphic.

Let \(G\) be a finite group, \(S\) (possibly, contains the identity element) be a subset of \(G\). The Bi-Cayley graph \(\text{BC}(G, S)\) is a bipartite graph with vertex set \(G \times \{0,1\}\) and edge set \(\{\{(g,0), (gs,1)\}, g \in G, s \in S\}\). A graph \(X\) is said to be super-edge-connected if every minimum edge cut of \(X\) is a set of edges incident with some vertex. The restricted edge connectivity \(\lambda'(X)\) of \(X\) is the minimum number of edges whose removal disconnects \(X\) into nontrivial components. A \(k\)-regular graph \(X\) is said to be optimally super-edge-connected if \(X\) is super-edge-connected and its restricted edge connectivity attains the maximum \(2k-2\). In this paper, we show that all connected Bi-Cayley graphs, except even cycles, are optimally super-edge-connected.