On Hamiltonian cycles in balanced $k$-partite graphs

Abstract

For all integers $k$ with $k\ge 2$, if $G$ is a balanced $k$-partite graph on $n\ge 3$ vertices with minimum degree at least \[ \left\lceil\frac{n}{2}\right\rceil+\left\lfloor\frac{n+2}{2\lceil\frac{k+1}{2}\rceil}\right\rfloor-\frac{n}{k}=\begin{cases} \lceil\frac{n}{2}\rceil+\lfloor\frac{n+2}{k+1}\rfloor-\frac{n}{k} & : k \text{ odd }\\ \frac{n}{2}+\lfloor\frac{n+2}{k+2}\rfloor-\frac{n}{k} & : k \text{ even } \end{cases}, \] then $G$ has a Hamiltonian cycle unless $k=2$ and 4 divides $n$, or $k=\frac{n}{2}$ and 4 divides $n$. In the case where $k=2$ and 4 divides $n$, or $k=\frac{n}{2}$ and 4 divides $n$, we can characterize the graphs which do not have a Hamiltonian cycle and see that $\lceil \frac{n}{2}\rceil +\lfloor \frac{n+2}{2\lceil \frac{k+1}{2}\rceil }\rfloor -\frac{n}{k}+1$ suffices. This result is tight for all $k\ge 2$ and $n\ge 3$ divisible by $k$.

14 pages, 2 figures. Minor updates