∀ {R : Type u_1} [inst : NormedRing R] {f g : ℕ → R},
  (Summable fun x => ‖f x‖) →
    Summable f →
      (Summable fun x => ‖g x‖) →
        Summable g → (∑' (n : ℕ), f n) * ∑' (n : ℕ), g n = ∑' (n : ℕ), ∑ k ∈ Finset.range (n + 1), f k * g (n - k)