%0 Journal Article %T An NP-hardness Result on the Monoid Frobenius Problem %A Zhi Xu %A J. Shallit %J Computer Science %D 2008 %I arXiv %X The following problem is NP-hard: given a regular expression $E$, decide if $E^*$ is not co-finite. %U http://arxiv.org/abs/0805.4049v3