Partial sums of geometric series in a ring

This file determines the values of the geometric series $\sum_{i=0}^{n-1} x^i$ and $\sum_{i=0}^{n-1} x^i y^{n-1-i}$ and variants thereof.

Several variants are recorded, generalising in particular to the case of a noncommutative ring in which x and y commute. Even versions not using division or subtraction, valid in each semiring, are recorded.

theorem geom_sum_succ {R : Type u_1} [Semiring R] {x : R} {n : ℕ} : ∑ i ∈ Finset.range (n + 1), x ^ i = x * ∑ i ∈ Finset.range n, x ^ i + 1

theorem geom_sum_succ' {R : Type u_1} [Semiring R] {x : R} {n : ℕ} : ∑ i ∈ Finset.range (n + 1), x ^ i = x ^ n + ∑ i ∈ Finset.range n, x ^ i

theorem geom_sum_zero {R : Type u_1} [Semiring R] (x : R) : ∑ i ∈ Finset.range 0, x ^ i = 0

theorem geom_sum_one {R : Type u_1} [Semiring R] (x : R) : ∑ i ∈ Finset.range 1, x ^ i = 1

@[simp]

theorem geom_sum_two {R : Type u_1} [Semiring R] {x : R} : ∑ i ∈ Finset.range 2, x ^ i = x + 1

@[simp]

theorem zero_geom_sum {R : Type u_1} [Semiring R] {n : ℕ} : ∑ i ∈ Finset.range n, 0 ^ i = if n = 0 then 0 else 1

theorem one_geom_sum {R : Type u_1} [Semiring R] (n : ℕ) : ∑ i ∈ Finset.range n, 1 ^ i = ↑n

theorem op_geom_sum {R : Type u_1} [Semiring R] (x : R) (n : ℕ) : MulOpposite.op (∑ i ∈ Finset.range n, x ^ i) = ∑ i ∈ Finset.range n, MulOpposite.op x ^ i

@[simp]

theorem op_geom_sum₂ {R : Type u_1} [Semiring R] (x y : R) (n : ℕ) : ∑ i ∈ Finset.range n, MulOpposite.op y ^ (n - 1 - i) * MulOpposite.op x ^ i = ∑ i ∈ Finset.range n, MulOpposite.op y ^ i * MulOpposite.op x ^ (n - 1 - i)

theorem geom_sum₂_with_one {R : Type u_1} [Semiring R] (x : R) (n : ℕ) : ∑ i ∈ Finset.range n, x ^ i * 1 ^ (n - 1 - i) = ∑ i ∈ Finset.range n, x ^ i

theorem Commute.geom_sum₂_mul_add {R : Type u_1} [Semiring R] {x y : R} (h : Commute x y) (n : ℕ) : (∑ i ∈ Finset.range n, (x + y) ^ i * y ^ (n - 1 - i)) * x + y ^ n = (x + y) ^ n

$x^n-y^n = (x-y) \sum x^ky^{n-1-k}$ reformulated without - signs.

theorem geom_sum₂_self {R : Type u_1} [Semiring R] (x : R) (n : ℕ) : ∑ i ∈ Finset.range n, x ^ i * x ^ (n - 1 - i) = ↑n * x ^ (n - 1)

theorem geom_sum_mul_add {R : Type u_1} [Semiring R] (x : R) (n : ℕ) : (∑ i ∈ Finset.range n, (x + 1) ^ i) * x + 1 = (x + 1) ^ n

theorem Commute.geom_sum₂_comm {R : Type u_1} [Semiring R] {x y : R} (n : ℕ) (h : Commute x y) : ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i) = ∑ i ∈ Finset.range n, y ^ i * x ^ (n - 1 - i)

theorem RingHom.map_geom_sum {R : Type u_1} {S : Type u_2} [Semiring R] [Semiring S] (x : R) (n : ℕ) (f : R →+* S) : f (∑ i ∈ Finset.range n, x ^ i) = ∑ i ∈ Finset.range n, f x ^ i

theorem RingHom.map_geom_sum₂ {R : Type u_1} {S : Type u_2} [Semiring R] [Semiring S] (x y : R) (n : ℕ) (f : R →+* S) : f (∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)) = ∑ i ∈ Finset.range n, f x ^ i * f y ^ (n - 1 - i)

theorem geom_sum₂_mul_add {R : Type u_1} [CommSemiring R] (x y : R) (n : ℕ) : (∑ i ∈ Finset.range n, (x + y) ^ i * y ^ (n - 1 - i)) * x + y ^ n = (x + y) ^ n

$x^n-y^n = (x-y) \sum x^ky^{n-1-k}$ reformulated without - signs.

theorem geom_sum₂_comm {R : Type u_1} [CommSemiring R] (x y : R) (n : ℕ) : ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i) = ∑ i ∈ Finset.range n, y ^ i * x ^ (n - 1 - i)

theorem geom_sum₂_mul_of_ge {R : Type u_1} [CommSemiring R] [PartialOrder R] [AddLeftReflectLE R] [AddLeftMono R] [ExistsAddOfLE R] [Sub R] [OrderedSub R] {x y : R} (hxy : y ≤ x) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)) * (x - y) = x ^ n - y ^ n

theorem geom_sum₂_mul_of_le {R : Type u_1} [CommSemiring R] [PartialOrder R] [AddLeftReflectLE R] [AddLeftMono R] [ExistsAddOfLE R] [Sub R] [OrderedSub R] {x y : R} (hxy : x ≤ y) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)) * (y - x) = y ^ n - x ^ n

theorem geom_sum_mul_of_one_le {R : Type u_1} [CommSemiring R] [PartialOrder R] [AddLeftReflectLE R] [AddLeftMono R] [ExistsAddOfLE R] [Sub R] [OrderedSub R] {x : R} (hx : 1 ≤ x) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i) * (x - 1) = x ^ n - 1

theorem geom_sum_mul_of_le_one {R : Type u_1} [CommSemiring R] [PartialOrder R] [AddLeftReflectLE R] [AddLeftMono R] [ExistsAddOfLE R] [Sub R] [OrderedSub R] {x : R} (hx : x ≤ 1) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i) * (1 - x) = 1 - x ^ n

@[simp]

theorem neg_one_geom_sum {R : Type u_1} [Ring R] {n : ℕ} : ∑ i ∈ Finset.range n, (-1) ^ i = if Even n then 0 else 1

theorem Commute.geom_sum₂_mul {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)) * (x - y) = x ^ n - y ^ n

theorem Commute.mul_neg_geom_sum₂ {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) (n : ℕ) : (y - x) * ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i) = y ^ n - x ^ n

theorem Commute.mul_geom_sum₂ {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) (n : ℕ) : (x - y) * ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i) = x ^ n - y ^ n

theorem Commute.sub_dvd_pow_sub_pow {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) (n : ℕ) : x - y ∣ x ^ n - y ^ n

theorem one_sub_dvd_one_sub_pow {R : Type u_1} [Ring R] (x : R) (n : ℕ) : 1 - x ∣ 1 - x ^ n

theorem sub_one_dvd_pow_sub_one {R : Type u_1} [Ring R] (x : R) (n : ℕ) : x - 1 ∣ x ^ n - 1

theorem pow_one_sub_dvd_pow_mul_sub_one {R : Type u_1} [Ring R] (x : R) (m n : ℕ) : x ^ m - 1 ∣ x ^ (m * n) - 1

theorem geom_sum_mul {R : Type u_1} [Ring R] (x : R) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i) * (x - 1) = x ^ n - 1

theorem mul_geom_sum {R : Type u_1} [Ring R] (x : R) (n : ℕ) : (x - 1) * ∑ i ∈ Finset.range n, x ^ i = x ^ n - 1

theorem geom_sum_mul_neg {R : Type u_1} [Ring R] (x : R) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i) * (1 - x) = 1 - x ^ n

theorem mul_neg_geom_sum {R : Type u_1} [Ring R] (x : R) (n : ℕ) : (1 - x) * ∑ i ∈ Finset.range n, x ^ i = 1 - x ^ n

theorem Commute.mul_geom_sum₂_Ico {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) {m n : ℕ} (hmn : m ≤ n) : (x - y) * ∑ i ∈ Finset.Ico m n, x ^ i * y ^ (n - 1 - i) = x ^ n - x ^ m * y ^ (n - m)

theorem Commute.geom_sum₂_succ_eq {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) {n : ℕ} : ∑ i ∈ Finset.range (n + 1), x ^ i * y ^ (n - i) = x ^ n + y * ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)

theorem Commute.geom_sum₂_Ico_mul {R : Type u_1} [Ring R] {x y : R} (h : Commute x y) {m n : ℕ} (hmn : m ≤ n) : (∑ i ∈ Finset.Ico m n, x ^ i * y ^ (n - 1 - i)) * (x - y) = x ^ n - y ^ (n - m) * x ^ m

theorem geom_sum_Ico_mul {R : Type u_1} [Ring R] (x : R) {m n : ℕ} (hmn : m ≤ n) : (∑ i ∈ Finset.Ico m n, x ^ i) * (x - 1) = x ^ n - x ^ m

theorem geom_sum_Ico_mul_neg {R : Type u_1} [Ring R] (x : R) {m n : ℕ} (hmn : m ≤ n) : (∑ i ∈ Finset.Ico m n, x ^ i) * (1 - x) = x ^ m - x ^ n

theorem geom_sum₂_mul {R : Type u_1} [CommRing R] (x y : R) (n : ℕ) : (∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)) * (x - y) = x ^ n - y ^ n

theorem sub_dvd_pow_sub_pow {R : Type u_1} [CommRing R] (x y : R) (n : ℕ) : x - y ∣ x ^ n - y ^ n

theorem Odd.add_dvd_pow_add_pow {R : Type u_1} [CommRing R] (x y : R) {n : ℕ} (h : Odd n) : x + y ∣ x ^ n + y ^ n

theorem geom_sum₂_succ_eq {R : Type u_1} [CommRing R] (x y : R) {n : ℕ} : ∑ i ∈ Finset.range (n + 1), x ^ i * y ^ (n - i) = x ^ n + y * ∑ i ∈ Finset.range n, x ^ i * y ^ (n - 1 - i)

theorem mul_geom_sum₂_Ico {R : Type u_1} [CommRing R] (x y : R) {m n : ℕ} (hmn : m ≤ n) : (x - y) * ∑ i ∈ Finset.Ico m n, x ^ i * y ^ (n - 1 - i) = x ^ n - x ^ m * y ^ (n - m)

theorem Nat.sub_dvd_pow_sub_pow (x y n : ℕ) : x - y ∣ x ^ n - y ^ n

@[deprecated Nat.sub_dvd_pow_sub_pow (since := "2025-08-23")]

theorem Nat.nat_sub_dvd_pow_sub_pow (x y n : ℕ) : x - y ∣ x ^ n - y ^ n

Alias of Nat.sub_dvd_pow_sub_pow.

theorem Nat.sub_one_dvd_pow_sub_one (x n : ℕ) : x - 1 ∣ x ^ n - 1

@[deprecated Nat.sub_one_dvd_pow_sub_one (since := "2025-08-23")]

theorem Nat.nat_pow_one_sub_dvd_pow_mul_sub_one (x n : ℕ) : x - 1 ∣ x ^ n - 1

Alias of Nat.sub_one_dvd_pow_sub_one.

theorem Nat.pow_sub_pow_dvd_pow_sub_pow {m k : ℕ} (x y : ℕ) (hmk : m ∣ k) : x ^ m - y ^ m ∣ x ^ k - y ^ k

theorem Nat.pow_sub_one_dvd_pow_sub_one {m k : ℕ} (x : ℕ) (hmk : m ∣ k) : x ^ m - 1 ∣ x ^ k - 1

theorem Odd.nat_add_dvd_pow_add_pow (x y : ℕ) {n : ℕ} (h : Odd n) : x + y ∣ x ^ n + y ^ n

theorem Nat.geomSum_eq {m : ℕ} (hm : 2 ≤ m) (n : ℕ) : ∑ k ∈ Finset.range n, m ^ k = (m ^ n - 1) / (m - 1)

Value of a geometric sum over the naturals. Note: see geom_sum_mul_add for a formulation that avoids division and subtraction.