ok... 1. The critic who said it was not right for me to argue about a free neutron has a point. However, nonrigorously+observationally speaking, nuclei with too many neutrons are unstable to beta decay, so that indicates neutronium should also be. (Nuclei with enough protons serving as "stable glue" can be stable, but then if you put even more protons they again become unstable because of inter-proton repulsion.) So, I won't pretend my derivation was rigorous, but I still believe it must be approximately right. 2. Re Chandrasekhar limit for white-dwarf collapse, I guess the question would be: is Chandrasekhar's reasoning reversible? That is, if M>Mchand, the dwarf collapses into a neutron star. If however magic aliens then removed some mass from the neutron star thus causing M<Mchand, will it then explode? I think the answer must be that there is some hysteresis. In fact due to energy release during the collapse, the neutron star will have less mass than the white dwarf, but it does not re-expand/explode. So it is not completely obvious what the "reverse threshold" ought to be, where a neutron star with mass below X, would explode. I think X<Mchand, not X=Mchand, but how much less, is not all that obvious. My derivation only gave what I have no doubt (despite criticism 1 above) is a very weak lower bound on X. 3. As somebody else pointed out, there are no magic alien mass-removal techniques, so who cares? Well, yes. That is why this is "math fun," I guess. Neutronium is not a stable substance in small amounts, and not even metastable enough for any use, so sci-fi writers should fuggedaboudit.