∀ {α : Type u_3} [inst : TopologicalSpace α] [inst_1 : NonUnitalNonAssocSemiring α] {f g : ℕ → α} [T3Space α]
  [IsTopologicalSemiring α],
  (Summable fun x => f x.1 * g x.2) → Summable fun n => ∑ k ∈ Finset.range (n + 1), f k * g (n - k)