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Gilbarg, David; Trudinger, Neil S. Elliptic partial differential equations of second order. Reprint of the 1998 edition.
Classics in Mathematics. Springer-Verlag, Berlin, 2001. xiv+517 pp. ISBN: 3-540-41160-7

Lieberman, Gary M. Second order parabolic differential equations.
World Scientific Publishing Co., Inc., River Edge, NJ, 1996. xii+439 pp. ISBN: 981-02-2883-X

Jost, Jürgen, Partial differential equations. Third edition.
Graduate Texts in Mathematics, 214. Springer, New York, 2013. xiv+410 pp. ISBN: 978-1-4614-4808-2; 978-1-4614-4809-9

DiBenedetto, Emmanuele, Partial differential equations. Second edition.
Cornerstones. Birkhäuser Boston, Inc., Boston, MA, 2010. xx+389 pp. ISBN: 978-0-8176-4551-9