We find finite, reasonably small, generator sets of the coordinate rings of
G-character varieties of finitely generated groups for all classical groups G.
This result together with the method of Grobner basis gives an algorithm for
describing character varieties by explicit polynomial equations.
Let G be a complex reductive algebraic group and let Gamma be a finitely
generated group. We study irreducible and completely reducible representations
rho: Gamma -> G in the context of the geometric invariant theory of the
G-action on Hom(Gamma,G) by conjugation.
Additionally, we study properties of character varieties, Hom(Gamma,G)//G. In
particular we describe the tangent spaces to X_G(Gamma) in terms of first
cohomology groups of Gamma with twisted coefficients, generalizing the well
known formula.