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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

3 votes
Accepted

Is there a useful theory of D-modules on smooth (non-analytic) manifolds?

Your « outsider » hunch is correct. Basically, the fact that there exist $C^\infty$ functions which are nowhere analytic is ruining the D-modules approach. It is in fact even worse: there exist $C^\in …
Bazin's user avatar
  • 16.2k
8 votes

PDEs and algebraic varieties

A most important result is missing in the previous answers, namely the characterization by Lars Hörmander of hypoellipticity in his seminal paper, On the theory of general partial differential operato …
Bazin's user avatar
  • 16.2k
3 votes
1 answer
210 views

Pythagorean number in Artin's theorem on nonnegative rational fractions

Emil Artin's theorem on nonnegative rational fractions says that a rational fraction $Q$ with $n$ variables with real coefficients which is non-negative on $\mathbb R^n$ is a sum of squares of rationa …