参考書

入門
熱・波動と微分方程式 (現代数学への入門)　俣野・神保
偏微分方程式論入門 (数学選書)　井川
偏微分方程式入門 (基礎数学)　金子
偏微分方程式 (共立数学講座 14)　熊之郷

中級
偏微分方程式論　溝畑
Partial Differential Equations (Graduate Studies in Mathematics, 19)　Evans
Partial Differential Equations (Applied Mathematical Sciences, 1)　john
Introduction to Partial Differential Equations. (Mathematical Notes, 102)　Folland
Partial Differential Equations I-Ⅲ: Basic Theory (Applied Mathematical Sciences, 115)　Taylor
Functional Analysis, Sobolev Spaces and Partial Differential Equations (Universitext) 　Brezis

上級
擬微分作用素　新開
擬微分作用素　熊之郷
The Analysis of Linear Partial Differential Operators I-Ⅳ: (Classics in Mathematics)　Hormander
Elliptic Partial Differential Equations of Second Order　Gilbarg, Trudinger