Differential operators
''The Atiyah-Patodi-Singer Index Theorem'' . Free online textbook.
''The Atiyah-Singer Index Theorem: What it is and why you should care'' . Pdf presentation.
''Invariance Theory, the Heat Equation, and the Atiyah–Singer Theorem'' . Free online textbook that proves the Atiyah–Singer theorem with a heat equation approach
Gilkey: Invariance Theory, ...
Article on Biharmonic Operator in MathWorld wolfram
Biharmonic Operator -- From MathWorld
Nonlinear Elliptic Equations at EqWorld: The World of Mathematical Equations.
Linear Elliptic Equations at EqWorld: The World of Mathematical Equations.
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