refactor(data/polynomial/degree/definitions): Remove hypothesis of nat_degree_X_pow_sub_C (#6628)

The lemma `nat_degree_X_pow_sub_C ` had an unnecessary hypothesis.

Author: tb65536

src/data/polynomial/degree/definitions.lean

@@ -866,9 +866,13 @@ theorem zero_nmem_multiset_map_X_sub_C {α : Type*} (m : multiset α) (f : α 
   (0 : polynomial R) ∉ m.map (λ a, X - C (f a)) :=
 λ mem, let ⟨a, _, ha⟩ := multiset.mem_map.mp mem in X_sub_C_ne_zero _ ha
 
-lemma nat_degree_X_pow_sub_C {n : ℕ} (hn : 0 < n) {r : R} :
+lemma nat_degree_X_pow_sub_C {n : ℕ} {r : R} :
   (X ^ n - C r).nat_degree = n :=
-by { apply nat_degree_eq_of_degree_eq_some, simp [degree_X_pow_sub_C hn], }
+begin
+  by_cases hn : n = 0,
+  { rw [hn, pow_zero, ←C_1, ←ring_hom.map_sub, nat_degree_C] },
+  { exact nat_degree_eq_of_degree_eq_some (degree_X_pow_sub_C (pos_iff_ne_zero.mpr hn) r) },
+end
 
 end nonzero_ring
 

src/ring_theory/polynomial/cyclotomic.lean

@@ -145,7 +145,7 @@ begin
   have hmonic : (X ^ n - C (1 : K)).monic := monic_X_pow_sub_C (1 : K) (ne_of_lt hpos).symm,
   symmetry,
   apply prod_multiset_X_sub_C_of_monic_of_roots_card_eq hmonic,
-  rw [@nat_degree_X_pow_sub_C K _ _ n hpos 1, ← nth_roots],
+  rw [@nat_degree_X_pow_sub_C K _ _ n 1, ← nth_roots],
   exact is_primitive_root.card_nth_roots h
 end
 
@@ -165,7 +165,6 @@ begin
   { simp only [hzero, ring_hom.map_one, splits_zero, pow_zero, sub_self] },
   rw [splits_iff_card_roots, ← nth_roots, is_primitive_root.card_nth_roots h,
     nat_degree_X_pow_sub_C],
-  exact nat.pos_of_ne_zero hzero
 end
 
 /-- If there is a primitive `n`-th root of unity in `K`, then