-- Mathlib/Analysis/MeanInequalities.lean
theorem rpow_sum_le_const_mul_sum_rpow (hp : 1 ≤ p) :
    (∑ i ∈ s, f i) ^ p ≤ (card s : ℝ≥0∞) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Proof state:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
have hp₁ : 1 / p * p = 1 := one_div_mul_cancel hpq.ne_zero

Result:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Proof state:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
hq : 1 / q * p = p - 1
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
simpa only [ENNReal.mul_rpow_of_nonneg _ _ hpq.nonneg, ← ENNReal.rpow_mul, hp₁, hq, coe_one,
  one_mul, one_rpow, rpow_one, Pi.one_apply, sum_const, Nat.smul_one_eq_cast] using
  ENNReal.rpow_le_rpow (inner_le_Lp_mul_Lq s 1 f hpq.symm) hpq.nonneg

Result:
no goals

Proof state:
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp : 1 ≤ p
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
rcases eq_or_lt_of_le hp with hp | hp

Result:
case inl
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 = p
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Proof state:
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
⊢ 1 / q * p = p - 1

Tactic:
rw [← hpq.div_conj_eq_sub_one]

Result:
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
⊢ 1 / q * p = p / q

Proof state:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
let q : ℝ := p / (p - 1)

Result:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Proof state:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
have hq : 1 / q * p = p - 1 := by
  rw [← hpq.div_conj_eq_sub_one]
  ring

Result:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
hq : 1 / q * p = p - 1
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Proof state:
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
hp₁ : 1 / p * p = 1
⊢ 1 / q * p = p / q

Tactic:
ring

Result:
no goals

Proof state:
case inl
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 = p
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
simp [← hp]

Result:
no goals

Proof state:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p

Tactic:
have hpq : p.IsConjExponent q := .conjExponent hp

Result:
case inr
ι : Type u
s : Finset ι
f : ι → ℝ≥0∞
p : ℝ
hp✝ : 1 ≤ p
hp : 1 < p
q : ℝ := p / (p - 1)
hpq : p.IsConjExponent q
⊢ (∑ i ∈ s, f i) ^ p ≤ ↑(#s) ^ (p - 1) * ∑ i ∈ s, f i ^ p