algebraic cone; monoid action; group action; orbit; action with unique fixed point; algebra automorphism; stable ideal; irreducible algebraic variety equidistribution of exponential sums on an arithmetic surface; \({\mathcal D}\)-modules; derived categories; differential algebra in the tannakian category; one-parameter families of exponential sums over finite fields; classical differential equations with polynomial coefficients; \(\ell \)- adic perverse sheaves; differential galois group; rigid GAGA; deformation equations 