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feat(ring_theory/integral_domain): generalize card_fiber_eq_of_mem_range
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…ange` * Generalize `card_fiber_eq_of_mem_range` to homomorphisms from a group to a monoid. * Rename it to `monoid_hom.card_fiber_eq_of_mem_range`, use `to_additive`.
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src/ring_theory/integral_domain.lean
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| @[to_additive] lemma monoid_hom.card_fiber_eq_of_mem_range {M} [monoid M] [decidable_eq M] | ||
| (f : G →* M) {x y : M} (hx : x ∈ set.range f) (hy : y ∈ set.range f) : | ||
| (univ.filter $ λ g, f g = x).card = (univ.filter $ λ g, f g = y).card := |
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We should probably make an equivalence f⁻¹ {x} ≃f.ker given x ∈ set.range f.
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…/mathlib into YK-range-lift
Co-authored-by: Anne Baanen <[email protected]>
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card_fiber_eq_of_mem_rangeto homomorphisms from a group to a monoid.monoid_hom.card_fiber_eq_of_mem_range, useto_additive.