mathlib3

472251b0 - feat(algebra): define linear recurrences and prove basic lemmas about them (#3829)

feat(algebra): define linear recurrences and prove basic lemmas about them (#3829) We define "linear recurrences", i.e assertions of the form `∀ n : ℕ, u (n + d) = a 0 * u n + a 1 * u (n+1) + ... + a (d-1) * u (n+d-1)`, and we introduce basic related lemmas and definitions (solution space, auxiliary polynomial). Currently, the most advanced theorem is : `q ^ n` is a solution iff `q` is a root of the auxiliary polynomial.