“…For perturbation theory, see [Aik1,AM,Anc4,5,Ban1,GH,HZ,M1,3,5,9,10,Pinc1,4,Pins2,Zhao]; as for estimates for fundamental solutions of parabolic equations, see [Aron,Bab,Ban1,2,Ciap,CoSa,Davis,DS,KS,Li,LY,M1,2,7,8,Nor1,2,Pinc2,Sulliv]; and as for general theory for positive solutions of elliptic/parabolic equations, see [Agmon,Anc3,CC,Helms,Phelps,Maeda,Pins2,Sulliv,Taylor].…”