apply finsum_cond_eq_sum_of_cond_iff f
R : Type u_1
inst✝ : AddCommMonoid R
f : ArithmeticFunction R
x : ℝ
⊢ ∑ᶠ (n : ℕ) (_ : ↑n < x), f n = ∑ n ∈ range ⌈x⌉₊, f n
R : Type u_1
inst✝ : AddCommMonoid R
f : ArithmeticFunction R
x : ℝ
⊢ ∀ {x_1 : ℕ}, f x_1 ≠ 0 → (↑x_1 < x ↔ x_1 ∈ range ⌈x⌉₊)