```
import Mathlib.Analysis.SpecialFunctions.Pow.Real
import Mathlib.Tactic
/-!
# International Mathematical Olympiad 2020, Problem 5
A deck of n > 1 cards is given. A positive integer is written on
each card. The deck has the property that the arithmetic mean of
the numbers on each pair of cards is also the geometric mean of
the numbers on some collection of one or more cards.
For which n does it follow that the numbers on the cards are all equal?
-/
namespace Imo2020P5
/- determine -/ abbrev SolutionSet : Set ℕ := sorry
noncomputable def geometric_mean {α : Type} (f : α → ℕ+) (s : Finset α) : ℝ :=
(∏ i ∈ s, (f i : ℝ))^((1:ℝ)/s.card)
theorem imo2020_p5 (n : ℕ) :
n ∈ SolutionSet ↔
(1 < n ∧
(∀ α : Type, [Fintype α] → Fintype.card α = n →
∀ f : α → ℕ+,
(∀ a b, ∃ s : Finset α,
geometric_mean f s = (((f a):ℝ) + f b) / 2)
→ ∃ y, ∀ a, f a = y )) := sorry
end Imo2020P5
```

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