mathlib

ebf8e67d - feat(linear_algebra/matrix): any matrix power can be expressed as sums of powers `0 ≤ k < fintype.card n`

feat(linear_algebra/matrix): any matrix power can be expressed as sums of powers `0 ≤ k < fintype.card n` I'm not familiar enoguh with the polynomial API to know if we can neatly get a similar result for negative powers.