We establish the nonexistence of:(i) Steiner \(t\)-\((v,k)\) trades of volume \(s\), for \(2^t + 2^{t-1} < s t+1\) and volume \(s < (t-1)2^t + 2\).

Using R. C. Read’s superposition method, we establish a formula for the enumeration of Euler multigraphs, with loops allowed and with given numbers of edges. In addition, applying Burnside’s Lemma and our adaptation of Read’s superposition method, we also derive a formula for the enumeration of Euler multigraphs without loops — via the calculation of the number of perfect matchings of the complement of complete multipartite graphs. MAPLE is employed to implement these enumerations. For one up to \(13\) edges, the numbers of nonisomorphic Euler multigraphs with loops allowed are:\(1, 3, 6, 16, 34, 90, 213, 572, 1499, 4231, 12115, 36660, 114105\) respectively, and for one up to \(16\) edges, the numbers of nonisomorphic Euler multigraphs without loops are:\(0, 1, 1, 4, 4, 15, 22, 68, 131, 376, 892, 2627, 7217, 22349, 69271, 229553\) respectively. Simplification of these methods yields the numbers of multigraphs with given numbers of edges, results which also appear to be new. Our methods also apply to multigraphs with essentially arbitrary constraints on vertex degrees.

In this paper, we determine the number of all maximal \(k\)-independent sets in the generalized lexicographical product of graphs. We construct a polynomial that calculates this number using the concept of Fibonacci polynomials and generalized Fibonacci polynomials. Also, for special graphs, we give the recurrence formula.

For a \(3\)-vertex coloring, a face of a triangulation whose vertices receive all three colors is called a vivid face with respect to it. In this paper, we show that for any triangulation \(G\) with \(n\) faces, there exists a coloring of \(G\) with at least \( \frac{1}{2}n\) faces and construct an infinite series of plane triangulations such that any \(3\)-coloring admits at most \(\frac{1}{5}(3n- 2)\) vivid faces.

A projective plane is equivalent to a disk with antipodal points identified. A graph is projective planar if it can be drawn on the projective plane with no crossing edges. A linear time algorithm for projective planar embedding has been described by Mohar. We provide a new approach that takes \(O(n^2)\) time but is much easier to implement. We programmed a variant of this algorithm and used it to computationally verify the known list of all the projective plane obstructions.

One application for this work is graph visualization. Projective plane embeddings can be represented on the plane and can provide aesthetically pleasing pictures of some non-planar graphs. More important is that it is highly likely that many problems that are computationally intractable (for example, NP-complete or #P-complete) have polynomial time algorithms when restricted to graphs of fixed orientable or non-orientable genus. Embedding the graph on the surface is likely to be the first step for these algorithms.

We consider the nonexistence of \(e\)-perfect codes in the Johnson scheme \(J(n, w)\). It is proved that for each \(J(2w + 3p, w)\) for \(p\) prime and \(p \neq 2, 5\), \(J(2w + 5p, w)\) for \(p\) prime and \(p \neq 3\), and \(J(2w + p^2, w)\) for \(p\) prime, it does not contain non-trivial \(e\)-perfect codes.

A graph \(G\) is called \(f\)-factor-covered if every edge of \(G\) is contained in some \(f\)-factor. \(G\) is called \(f\)-factor-deleted if \(G\) – \(e\) contains an \(f\)-factor for every edge \(e\). Babler proved that every \(r\)-regular, \((r – 1)\)-edge-connected graph of even order has a \(1\)-factor. In the present article, we prove that every \(2r\)-regular graph of odd order is both \(2m\)-factor-covered and \(2m\)-factor-deleted for all integers \(m\), \(1 \leq m \leq r – 1\), and every \(r\)-regular, \((r – 1)\)-edge-connected graph of even order is both \(m\)-factor-covered and \(m\)-factor-deleted for all integers \(m\), \(1 \leq m \leq \left\lfloor \frac{r}{2} \right\rfloor\).

The convex hull of a subset \(A\) of \(V(G)\), where \(G\) is a connected graph, is defined as the smallest convex set in \(G\) containing \(A\). The hull number of \(G\) is the cardinality of a smallest set \(A\) whose convex hull is \(V(G)\). In this paper, we give the hull number of the composition of two connected graphs.

The basis number \(b(G)\) of a graph \(G\) is defined to be the least integer \(d\) such that \(G\) has a \(d\)-fold basis for its cycle space. In this paper, we investigate the basis number of the direct product of theta graphs and paths.