Reverse of a univariate polynomial

The main definition is reverse. Applying reverse to a polynomial f : R[X] produces the polynomial with a reversed list of coefficients, equivalent to X^f.natDegree * f(1/X).

The main result is that reverse (f * g) = reverse f * reverse g, provided the leading coefficients of f and g do not multiply to zero.

def Polynomial.revAtFun (N : ℕ) (i : ℕ) : ℕ

If i ≤ N, then revAtFun N i returns N - i, otherwise it returns i. This is the map used by the embedding revAt.

Equations

• Polynomial.revAtFun N i = if i ≤ N then N - i else i

theorem Polynomial.revAtFun_invol {N : ℕ} {i : ℕ} : Polynomial.revAtFun N (Polynomial.revAtFun N i) = i

theorem Polynomial.revAtFun_inj {N : ℕ} : Function.Injective (Polynomial.revAtFun N)

def Polynomial.revAt (N : ℕ) : ℕ ↪ ℕ

If i ≤ N, then revAt N i returns N - i, otherwise it returns i. Essentially, this embedding is only used for i ≤ N. The advantage of revAt N i over N - i is that revAt is an involution.

Equations

• Polynomial.revAt N = { toFun := fun (i : ℕ) => if i ≤ N then N - i else i, inj' := ⋯ }

@[simp]

theorem Polynomial.revAtFun_eq (N : ℕ) (i : ℕ) : Polynomial.revAtFun N i = (Polynomial.revAt N) i

We prefer to use the bundled revAt over unbundled revAtFun.

@[simp]

theorem Polynomial.revAt_invol {N : ℕ} {i : ℕ} : (Polynomial.revAt N) ((Polynomial.revAt N) i) = i

@[simp]

theorem Polynomial.revAt_le {N : ℕ} {i : ℕ} (H : i ≤ N) : (Polynomial.revAt N) i = N - i

theorem Polynomial.revAt_eq_self_of_lt {N : ℕ} {i : ℕ} (h : N < i) : (Polynomial.revAt N) i = i

theorem Polynomial.revAt_add {N : ℕ} {O : ℕ} {n : ℕ} {o : ℕ} (hn : n ≤ N) (ho : o ≤ O) : (Polynomial.revAt (N + O)) (n + o) = (Polynomial.revAt N) n + (Polynomial.revAt O) o

theorem Polynomial.revAt_zero (N : ℕ) : (Polynomial.revAt N) 0 = N

noncomputable def Polynomial.reflect {R : Type u_1} [Semiring R] (N : ℕ) : Polynomial R → Polynomial R

reflect N f is the polynomial such that (reflect N f).coeff i = f.coeff (revAt N i). In other words, the terms with exponent [0, ..., N] now have exponent [N, ..., 0].

In practice, reflect is only used when N is at least as large as the degree of f.

Eventually, it will be used with N exactly equal to the degree of f.

Equations

• Polynomial.reflect N x = match x with | { toFinsupp := f } => { toFinsupp := Finsupp.embDomain (Polynomial.revAt N) f }

theorem Polynomial.reflect_support {R : Type u_1} [Semiring R] (N : ℕ) (f : Polynomial R) : (Polynomial.reflect N f).support = Finset.image (⇑(Polynomial.revAt N)) f.support

@[simp]

theorem Polynomial.coeff_reflect {R : Type u_1} [Semiring R] (N : ℕ) (f : Polynomial R) (i : ℕ) : (Polynomial.reflect N f).coeff i = f.coeff ((Polynomial.revAt N) i)

@[simp]

theorem Polynomial.reflect_zero {R : Type u_1} [Semiring R] {N : ℕ} : Polynomial.reflect N 0 = 0

@[simp]

theorem Polynomial.reflect_eq_zero_iff {R : Type u_1} [Semiring R] {N : ℕ} {f : Polynomial R} : Polynomial.reflect N f = 0 ↔ f = 0

@[simp]

theorem Polynomial.reflect_add {R : Type u_1} [Semiring R] (f : Polynomial R) (g : Polynomial R) (N : ℕ) : Polynomial.reflect N (f + g) = Polynomial.reflect N f + Polynomial.reflect N g

@[simp]

theorem Polynomial.reflect_C_mul {R : Type u_1} [Semiring R] (f : Polynomial R) (r : R) (N : ℕ) : Polynomial.reflect N (Polynomial.C r * f) = Polynomial.C r * Polynomial.reflect N f

theorem Polynomial.reflect_C_mul_X_pow {R : Type u_1} [Semiring R] (N : ℕ) (n : ℕ) {c : R} : Polynomial.reflect N (Polynomial.C c * Polynomial.X ^ n) = Polynomial.C c * Polynomial.X ^ (Polynomial.revAt N) n

@[simp]

theorem Polynomial.reflect_C {R : Type u_1} [Semiring R] (r : R) (N : ℕ) : Polynomial.reflect N (Polynomial.C r) = Polynomial.C r * Polynomial.X ^ N

@[simp]

theorem Polynomial.reflect_monomial {R : Type u_1} [Semiring R] (N : ℕ) (n : ℕ) : Polynomial.reflect N (Polynomial.X ^ n) = Polynomial.X ^ (Polynomial.revAt N) n

@[simp]

theorem Polynomial.reflect_one_X {R : Type u_1} [Semiring R] : Polynomial.reflect 1 Polynomial.X = 1

theorem Polynomial.reflect_mul_induction {R : Type u_1} [Semiring R] (cf : ℕ) (cg : ℕ) (N : ℕ) (O : ℕ) (f : Polynomial R) (g : Polynomial R) : f.support.card ≤ cf.succ → g.support.card ≤ cg.succ → f.natDegree ≤ N → g.natDegree ≤ O → Polynomial.reflect (N + O) (f * g) = Polynomial.reflect N f * Polynomial.reflect O g

@[simp]

theorem Polynomial.reflect_mul {R : Type u_1} [Semiring R] (f : Polynomial R) (g : Polynomial R) {F : ℕ} {G : ℕ} (Ff : f.natDegree ≤ F) (Gg : g.natDegree ≤ G) : Polynomial.reflect (F + G) (f * g) = Polynomial.reflect F f * Polynomial.reflect G g

theorem Polynomial.eval₂_reflect_mul_pow {R : Type u_1} [Semiring R] {S : Type u_2} [CommSemiring S] (i : R →+* S) (x : S) [Invertible x] (N : ℕ) (f : Polynomial R) (hf : f.natDegree ≤ N) : Polynomial.eval₂ i (⅟x) (Polynomial.reflect N f) * x ^ N = Polynomial.eval₂ i x f

theorem Polynomial.eval₂_reflect_eq_zero_iff {R : Type u_1} [Semiring R] {S : Type u_2} [CommSemiring S] (i : R →+* S) (x : S) [Invertible x] (N : ℕ) (f : Polynomial R) (hf : f.natDegree ≤ N) : Polynomial.eval₂ i (⅟x) (Polynomial.reflect N f) = 0 ↔ Polynomial.eval₂ i x f = 0

noncomputable def Polynomial.reverse {R : Type u_1} [Semiring R] (f : Polynomial R) : Polynomial R

The reverse of a polynomial f is the polynomial obtained by "reading f backwards". Even though this is not the actual definition, reverse f = f (1/X) * X ^ f.natDegree.

Equations

• f.reverse = Polynomial.reflect f.natDegree f

theorem Polynomial.coeff_reverse {R : Type u_1} [Semiring R] (f : Polynomial R) (n : ℕ) : f.reverse.coeff n = f.coeff ((Polynomial.revAt f.natDegree) n)

@[simp]

theorem Polynomial.coeff_zero_reverse {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.coeff 0 = f.leadingCoeff

@[simp]

theorem Polynomial.reverse_zero {R : Type u_1} [Semiring R] : Polynomial.reverse 0 = 0

@[simp]

theorem Polynomial.reverse_eq_zero {R : Type u_1} [Semiring R] {f : Polynomial R} : f.reverse = 0 ↔ f = 0

theorem Polynomial.reverse_natDegree_le {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.natDegree ≤ f.natDegree

theorem Polynomial.natDegree_eq_reverse_natDegree_add_natTrailingDegree {R : Type u_1} [Semiring R] (f : Polynomial R) : f.natDegree = f.reverse.natDegree + f.natTrailingDegree

theorem Polynomial.reverse_natDegree {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.natDegree = f.natDegree - f.natTrailingDegree

theorem Polynomial.reverse_leadingCoeff {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.leadingCoeff = f.trailingCoeff

theorem Polynomial.natTrailingDegree_reverse {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.natTrailingDegree = 0

theorem Polynomial.reverse_trailingCoeff {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.trailingCoeff = f.leadingCoeff

theorem Polynomial.reverse_mul {R : Type u_1} [Semiring R] {f : Polynomial R} {g : Polynomial R} (fg : f.leadingCoeff * g.leadingCoeff ≠ 0) : (f * g).reverse = f.reverse * g.reverse

@[simp]

theorem Polynomial.reverse_mul_of_domain {R : Type u_2} [Ring R] [NoZeroDivisors R] (f : Polynomial R) (g : Polynomial R) : (f * g).reverse = f.reverse * g.reverse

theorem Polynomial.trailingCoeff_mul {R : Type u_2} [Ring R] [NoZeroDivisors R] (p : Polynomial R) (q : Polynomial R) : (p * q).trailingCoeff = p.trailingCoeff * q.trailingCoeff

@[simp]

theorem Polynomial.coeff_one_reverse {R : Type u_1} [Semiring R] (f : Polynomial R) : f.reverse.coeff 1 = f.nextCoeff

@[simp]

theorem Polynomial.reverse_C {R : Type u_1} [Semiring R] (t : R) : (Polynomial.C t).reverse = Polynomial.C t

@[simp]

theorem Polynomial.reverse_mul_X {R : Type u_1} [Semiring R] (p : Polynomial R) : (p * Polynomial.X).reverse = p.reverse

@[simp]

theorem Polynomial.reverse_X_mul {R : Type u_1} [Semiring R] (p : Polynomial R) : (Polynomial.X * p).reverse = p.reverse

@[simp]

theorem Polynomial.reverse_mul_X_pow {R : Type u_1} [Semiring R] (p : Polynomial R) (n : ℕ) : (p * Polynomial.X ^ n).reverse = p.reverse

@[simp]

theorem Polynomial.reverse_X_pow_mul {R : Type u_1} [Semiring R] (p : Polynomial R) (n : ℕ) : (Polynomial.X ^ n * p).reverse = p.reverse

@[simp]

theorem Polynomial.reverse_add_C {R : Type u_1} [Semiring R] (p : Polynomial R) (t : R) : (p + Polynomial.C t).reverse = p.reverse + Polynomial.C t * Polynomial.X ^ p.natDegree

@[simp]

theorem Polynomial.reverse_C_add {R : Type u_1} [Semiring R] (p : Polynomial R) (t : R) : (Polynomial.C t + p).reverse = Polynomial.C t * Polynomial.X ^ p.natDegree + p.reverse

theorem Polynomial.eval₂_reverse_mul_pow {R : Type u_1} [Semiring R] {S : Type u_2} [CommSemiring S] (i : R →+* S) (x : S) [Invertible x] (f : Polynomial R) : Polynomial.eval₂ i (⅟x) f.reverse * x ^ f.natDegree = Polynomial.eval₂ i x f

@[simp]

theorem Polynomial.eval₂_reverse_eq_zero_iff {R : Type u_1} [Semiring R] {S : Type u_2} [CommSemiring S] (i : R →+* S) (x : S) [Invertible x] (f : Polynomial R) : Polynomial.eval₂ i (⅟x) f.reverse = 0 ↔ Polynomial.eval₂ i x f = 0

@[simp]

theorem Polynomial.reflect_neg {R : Type u_1} [Ring R] (f : Polynomial R) (N : ℕ) : Polynomial.reflect N (-f) = -Polynomial.reflect N f

@[simp]

theorem Polynomial.reflect_sub {R : Type u_1} [Ring R] (f : Polynomial R) (g : Polynomial R) (N : ℕ) : Polynomial.reflect N (f - g) = Polynomial.reflect N f - Polynomial.reflect N g

@[simp]

theorem Polynomial.reverse_neg {R : Type u_1} [Ring R] (f : Polynomial R) : (-f).reverse = -f.reverse