1. Aronson, D.G.: Non-negative solutions of linear parabolic equations. Ann. Sc. Norm. Sup. Pisa (3) 22, 607–694 (1968)
2. Birman, M.S.: On the spectrum of singular boundary-value problems. Mat. Sbornik 55(97), 125–174 (1961) (in Russian)
3. Davies, E.B.: Pointwise bounds on the space and time derivatives of heat kernels. J. Oper. Theory 21, 367–378 (1989)
4. De Giorgi, E.: Sulla differenziabilità e l’analiticità delle estremali degli integrali multipli regolari. Mem. Acc. Sci. Torino 3, 25–43 (1957)
5. Fabes, E.B., Stroock, D.W.: A new proof of Moser’s parabolic Harnack inequality via the old ideas of Nash. Arch. Ration. Mech. Anal. 96, 327–338 (1986)