L. C. Evans, Partial Differential Equations , Graduate Studies in Mathematics, Volume 19, AMS.
Errata for the book may be found on Craig Evans' homepage .
Some other references are the following.
For a detailed account of Sobolev spaces, see:
R. A. Adams, Sobolev Spaces , Academic Press, New York, 1975.
For more on elliptic PDEs see:
D. Gilbarg, and N. S. Trudinger, Elliptic Partial Differential Equations of Second Order , Springer-Verlag, New York, 1983.
A briefer discussion of linear and nonlinear elliptic PDEs, similar to the discussion in Evans, is in Chapters 8 and 9 of:
M. Renardy, and R. Rogers, An Introduction to Partial Differential Equations , Springer-Verlag, New York, 1993.
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