have h1 := nicomachus (n + 1)
n : ℕ
⊢ ∑ x ∈ Finset.range n, (x + 1) ^ 3 = (∑ x ∈ Finset.range n, (x + 1)) ^ 2
n : ℕ
h1 : ∑ i ∈ Finset.range (n + 1), i ^ 3 = (∑ i ∈ Finset.range (n + 1), i) ^ 2
⊢ ∑ x ∈ Finset.range n, (x + 1) ^ 3 = (∑ x ∈ Finset.range n, (x + 1)) ^ 2