-- Mathlib/NumberTheory/AbelSummation.lean
theorem sum_mul_eq_sub_integral_mul₀ (hc : c 0 = 0) (b : ℝ)
    (hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t)
    (hf_int : IntegrableOn (deriv f) (Set.Icc 1 b)) :
    ∑ k ∈ Icc 0 ⌊b⌋₊, f k * c k =
      f b * (∑ k ∈ Icc 0 ⌊b⌋₊, c k) - ∫ t in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Proof state:
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
obtain hb | hb := le_or_gt 1 b

Result:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

case inr
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 > b
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Proof state:
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ Nat.succ 0 ≤ ⌊b⌋₊

Tactic:
omega

Result:
no goals

Proof state:
case inr
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 > b
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
simp_rw [Nat.floor_eq_zero.mpr hb, Icc_self, sum_singleton, Nat.cast_zero, hc, mul_zero,
  Set.Ioc_eq_empty_of_le hb.le, Measure.restrict_empty, integral_zero_measure, sub_self]

Result:
no goals

Proof state:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ f ↑0 * c 0 + (f ↑(Nat.succ 0) * c (Nat.succ 0) + ∑ x ∈ Ioc (Nat.succ 0) ⌊b⌋₊, f ↑x * c x) =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
rw [Nat.succ_eq_add_one, zero_add, ← Nat.floor_one (α := ℝ),
  sum_mul_eq_sub_sub_integral_mul c zero_le_one hb hf_diff hf_int, Nat.floor_one, Nat.cast_one,
  Icc_eq_cons_Ioc zero_le_one, sum_cons, show 1 = 0 + 1 by rfl, Nat.Ioc_succ_singleton,
  zero_add, sum_singleton, hc, mul_zero, zero_add]

Result:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ f 1 * c 1 +
      (f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - f 1 * (0 + c 1) - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k) =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Proof state:
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ 1 = 0 + 1

Tactic:
rfl

Result:
no goals

Proof state:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ f 1 * c 1 +
      (f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - f 1 * (0 + c 1) - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k) =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
ring

Result:
no goals

Proof state:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
have : 1 ≤ ⌊b⌋₊ := (Nat.one_le_floor_iff _).mpr hb

Result:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Proof state:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ ∑ k ∈ Icc 0 ⌊b⌋₊, f ↑k * c k =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k

Tactic:
nth_rewrite 1 [Icc_eq_cons_Ioc (Nat.zero_le _), sum_cons, ← Nat.Icc_succ_left,
  Icc_eq_cons_Ioc (by omega), sum_cons]

Result:
case inl
𝕜 : Type u_1
inst✝ : RCLike 𝕜
c : ℕ → 𝕜
f : ℝ → 𝕜
hc : c 0 = 0
b : ℝ
hf_diff : ∀ t ∈ Set.Icc 1 b, DifferentiableAt ℝ f t
hf_int : IntegrableOn (deriv f) (Set.Icc 1 b) volume
hb : 1 ≤ b
this : 1 ≤ ⌊b⌋₊
⊢ f ↑0 * c 0 + (f ↑(Nat.succ 0) * c (Nat.succ 0) + ∑ x ∈ Ioc (Nat.succ 0) ⌊b⌋₊, f ↑x * c x) =
    f b * ∑ k ∈ Icc 0 ⌊b⌋₊, c k - ∫ (t : ℝ) in Set.Ioc 1 b, deriv f t * ∑ k ∈ Icc 0 ⌊t⌋₊, c k