5 “Infeasibility” is a noun and “unfeasible”, as nan grey notes, is definitely an adjective herisson KeldericKelderic 47211 gold badge44 silver badges88 bronze badges ten You should provide proof. Long ago, considered one of my academics advocated "none is", so I use that (apart from often I do not). Ngram https://altonp520hqy7.eveowiki.com/user