  Mehring et al. have recently described an elegant nuclear magnetic resonance
(NMR) experiment implementing an algorithm to factor numbers based on the
properties of Gauss sums. Similar experiments have also been described by
Mahesh et al. In fact these algorithms do not factor numbers directly, but
rather check whether a trial integer $\ell$ is a factor of a given integer $N$.
Here I show that these NMR schemes cannot be used for factor checking without
first implicitly determining whether or not $\ell$ is a factor of $N$.
