feat(algebra/periodic): `const_add`, `add_const`, `const_sub`, `sub_const` (#15782)
feat(algebra/periodic): `const_add`, `add_const`, `const_sub`, `sub_const` (#15782)

Add lemmas that if `f` is periodic (or antiperiodic), so are functions
such as `λ x, f (a + x)` and `λ x, f (x - a)` where a constant is
added or subtracted on either side of the argument, with the same
period.  As far as I can tell, while mathlib has such lemmas about the
effect of multiplying or dividing the argument by a constant on the
period, it doesn't have them for addition or subtraction.

It's possible some of these lemmas are true under weaker type class
assumptions, though I think the type classes used are at least minimal
for the proofs I used.







leanprover-community/mathlib,leanprover-community/mathlib