Pancyclicity, Panconnectivity, and Panpositionability for General Graphs and Bipartite Graphs

Abstract

A graph $G$ is pancyclic if it contains a cycle of every length from 3 to $|V(G)|$ inclusive. A graph $G$ is panconnected if there exists a path of length $l$ joining any two different vertices $x$ and $y$ with $d_G(x,y)\le l\le |V(G)|–1$, where $d_G(x,y)$ denotes the distance between $x$ and $y$ in $G$. A hamiltonian graph $G$ is panpositionable if for any two different vertices $x$ and $y$ of $G$ and any integer $k$ with $d_G(x,y)\le k\le |V(G)|/2$, there exists a hamiltonian cycle $C$ of $G$ with $d_C(x,y)=k$, where $d_C(x,y)$ denotes the distance between $x$ and $y$ in a hamiltonian cycle $C$ of $G$. It is obvious that panconnected graphs are pancyclic, and panpositionable graphs are pancyclic.

The above properties can be studied in bipartite graphs after some modification. A graph $H=(V_0\cup V_1,E)$ is bipartite if $V(H)=V_0\cup V_1$ and $E(H)$ is a subset of $\{(u,v)|u\in V_0\text{ and }v\in V_1\}$. A graph is bipancyclic if it contains a cycle of every even length from 4 to $2\lfloor |V(H)|/2\rfloor$ inclusive. A graph $H$ is bipanconnected if there exists a path of length $l$ joining any two different vertices $x$ and $y$ with $d_H(x,y)\le l\le |V(H)|–1$, where $d_H(x,y)$ denotes the distance between $x$ and $y$ in $H$ and $l–d_H(x,y)$ is even. A hamiltonian graph $H$ is bipanpositionable if for any two different vertices $x$ and $y$ of $H$ and for any integer $k$ with $d_H(x,y)\le k\le |V(H)|/2$, there exists a hamiltonian cycle $C$ of $H$ with $d_C(x,y)=k$, where $d_C(x,y)$ denotes the distance between $x$ and $y$ in a hamiltonian cycle $C$ of $H$ and $k–d_H(x,y)$ is even. It can be shown that bipanconnected graphs are bipancyclic, and bipanpositionable graphs are bipancyclic.

In this paper, we present some examples of pancyclic graphs that are neither panconnected nor panpositionable, some examples of panconnected graphs that are not panpositionable, and some examples of graphs that are panconnected and panpositionable, for nonbipartite graphs. Corresponding examples for bipartite graphs are discussed. The existence of panpositionable (or bipanpositionable, resp.) graphs that are not panconnected (or bipanconnected, resp.) is still an open problem.