Let \(G\) be a simple graph. The double vertex graph \(U_2(G)\) of \(G\) is the graph whose vertex set consists of all \(2\)-subsets of \(V(G)\) such that two distinct vertices \(\{x,y\}\) and \(\{u,v\}\) are adjacent if and only if \(|\{x,y\} \cap \{u,v\}| = 1\) and if \(x = u\), then \(y\) and \(v\) are adjacent in \(G\). In this paper, we consider the exponents and primitivity relationships between a simple graph and its double vertex graph. A sharp upper bound on exponents of double vertex graphs of primitive simple graphs and the characterization of extremal graphs are obtained.

Let \(S_n\) be the set of permutations on \(\{1, \ldots, n\}\) and \(\pi \in S_n\). Let \(d(\pi)\) be the arithmetic average of \(\{|i – \pi(i)| : 1 \leq i \leq n\}\). Then \(d(\pi)/n \in [0, 1/2]\), the expected value of \(d(\pi)/n\) approaches \(1/3\) as \(n\) approaches infinity, and \(d(\pi)/n\) is close to \(1/3\) for most permutations. We describe all permutations \(\pi\) with maximal \(d(\pi)\).

Let \(s^+(\pi)\) and \(s^*(\pi)\) be the arithmetic and geometric averages of \(\{|\pi(i) – \pi(i + 1)| : 1 \leq i 1\). We describe all permutations \(\pi\),\(\sigma\) with maximal \(s^+(\pi)\) and \(s^*(\sigma)\).

A connected graph \(G = (V,E)\) is said to be \((a,d)\)-antimagic, for some positive integers \(a\) and \(d\), if its edges admit a labeling by all the integers in the set \(\{1, 2, \ldots, |E(G)|\}\) such that the induced vertex labels, obtained by adding all the labels of the edges adjacent to each vertex, consist of an arithmetic progression with the first term \(a\) and the common difference \(d\). Mirka Miller and Martin Bača proved that the generalized Petersen graph \(P(n,2)\) is \((\frac{3n+6}{2}, 3)\)-antimagic for \(n \equiv 0 \pmod{4}\), \(n \geq 8\), and conjectured that \(P(n, k)\) is \((\frac{5n+5}{2}, 2)\)-antimagic for odd \(n\) and \(2 \leq k \leq \frac{n}{2}-1\). In this paper, we show that the generalized Petersen graph \(P(n,2)\) is \((\frac{5n+5}{2}, 2)\)-antimagic for \(n \equiv 3 \pmod{4}\), \(n \geq 7\).

Sierpiński graphs \(S(n,k)\), \(n, k \in \mathbb{N}\), can be interpreted as graphs of a variant of the Tower of Hanoi with \(k \geq 3\) pegs and \(n \geq 1\) discs. In particular, it has been proved that for \(k = 3\) the graphs \(S(n, 3)\) are isomorphic to the Hanoi graphs \(H_3^n\). In this paper, we will determine the chromatic number, the diameter, the eccentricity of a vertex, the radius, and the centre of \(S(n,k)\). Moreover, we will derive an important invariant and a number-theoretical characterization of \(S(n,k)\). By means of these results, we will determine the complexity of Problem \(1\), that is, the complexity of getting from an arbitrary vertex \(v \in S(n,k)\) to the nearest and to the most distant extreme vertex. For the Hanoi graphs \(H_3^n\), some of these results are new.

In this paper, we will prove that there exist no \([n,k,d]_q\) codes of \(sq^{k-1}-(s+t)q^{k-2}-q^{k-4} \leq d \leq sq^{k-1}-(s+t)q^{k-2}\) attaining the Griesmer bound with \(k \geq 4, 1 \leq s \leq k-2, t \geq 1\), and \(s+t \leq (q+1)\backslash 2\). Furthermore, we will prove that there exist no \([n,k,d]_q\) codes for \(sq^{k-1}-(s+t)q^{k-2}-q^{k-3} \leq d \leq s\) attaining the Griesmer bound with \(k \geq 3\), \(1 \leq s \leq k-2\), \(t \geq 1\), and \(s+t \leq \sqrt{q}-1\). The results generalize the nonexistence theorems of Tatsuya Maruta (see \([7]\)) and Andreas Klein (see \([4]\)) to a larger class of codes.

We study the spectral radius of graphs with \(n\) vertices and a \(k\)-vertex cut and describe the graph which has the maximal spectral radius in this class. We also discuss the limit point of the maximal spectral radius.

Consider lattice paths in \(\mathbb{Z}^2\) taking unit steps north (N) and east (E). Fix positive integers \(r,s\) and put an equivalence relation on points of \(\mathbb{Z}^2\) by letting \(v,w\) be equivalent if \(v-w = \ell(r,s)\) for some \(k \in \mathbb{Z}\). Call a lattice path \({valid}\) if whenever it enters a point \(v\) with an E-step, then any further points of the path in the equivalence class of \(v\) are also entered with an E-step. Loehr and Warrington conjectured that the number of valid paths from \((0,0)\) to \((nr,ns)\) is \({\binom{r+s}{nr}}^n\). We prove this conjecture when \(s=2\).

Given integers \(m \geq 2, r \geq 2\), let \(q_m(n), q_0^{(m)}(n), b_r^{(m)}(n)\) denote respectively the number of \(m\)-colored partitions of \(n\) into: distinct parts, distinct odd parts, and parts not divisible by \(r\).We obtain recurrences for each of the above-mentioned types of partition functions.

A reflection of a regular map on a Riemann surface fixes some simple closed curves, which are called \({mirrors}\). Each mirror passes through some of the geometric points (vertices, face-centers and edge-centers) of the map such that these points form a periodic sequence which we call the \({pattern}\) of the mirror. For every mirror there exist two particular conformal automorphisms of the map that fix the mirror setwise and rotate it in opposite directions. We call these automorphisms the \({rotary\; automorphisms}\) of the mirror. In this paper, we first introduce the notion of pattern and then describe the patterns of mirrors on surfaces. We also determine the rotary automorphisms of mirrors. Finally, we give some necessary conditions under which all reflections of a regular map are conjugate.