apply sum_congr rfl
s : SelbergSieve
⊢ ∑ l ∈ P.divisors, 1 / g l * (∑ d ∈ P.divisors, if l ∣ d then ν d * s.selbergWeights d else 0) ^ 2 =
    ∑ l ∈ P.divisors, if ↑l ^ 2 ≤ y then g l * S⁻¹ ^ 2 else 0
s : SelbergSieve
⊢ ∀ x ∈ P.divisors,
    1 / g x * (∑ d ∈ P.divisors, if x ∣ d then ν d * s.selbergWeights d else 0) ^ 2 =
      if ↑x ^ 2 ≤ y then g x * S⁻¹ ^ 2 else 0