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The question is as follows:

Are some good necessary and/or sufficient conditions known for when can $K_{m,n}$ be factorized into Hamiltonian paths?

In other words when can we have an edge disjoint union of Hamiltonian paths equaling the graph $K_{m,n}$.

The analogous solution for the problem for Hamiltonian cycles is well known (such graphs are characterized by $m=n=2k>1$). I searched the internet for a solution for the analogous path problem to no avail. I suspect that it is not known.

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Suppose without loss of generality that $m \geq n$. Then either $m = n$ or $m = n + 1$.

If $m = n$, then each Hamiltonian path in $K_{m,m}$ has $2m-1$ edges. If there are $h$ paths, then $h(2m-1) = m^2$, which is impossible, since $2m-1$ has prime factors distinct from those of $m$ while $m^2$ does not.

If $m = n+1$, then each Hamiltonian path in $K_{n+1,n}$ has $2n$ edges. If there are $h$ paths, then $h(2n) = n(n+1)$, so $h = \frac{n+1}{2}$.

Hence we must have $n+1$ even, and indeed, all graphs $K_{2k,2k-1}$ can be decomposed into Hamiltonian paths. To see this, consider adding a vertex to the smaller side; then there is a decomposition into Hamiltonian cycles; removing the extra vertex leaves a decomposition of our original graph into Hamiltonian paths.

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  • $\begingroup$ Thank you. I understand your answer. Could you give a reference for it as well or this is your own solution? $\endgroup$ – Shahab Feb 22 '16 at 3:13
  • $\begingroup$ @Shahab: It's my own solution. $\endgroup$ – Nick Matteo Feb 22 '16 at 3:20
  • $\begingroup$ May I cite it in your name as given on your website? $\endgroup$ – Shahab Feb 22 '16 at 3:26
  • $\begingroup$ @Shahab: Sure! (Or feel free to use the result without citing me.) $\endgroup$ – Nick Matteo Feb 22 '16 at 3:52

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