Sum of the nonnegative integers below 991

Proved

FiniteTriangular.sum_range_991

by carlok · Sep 26, 2026 · Mathlib 0df444a (Lean v4.33.1)

algebrafinset

The sum of the integers from $0$ through $990$ equals $490545$ , which is the triangular number $\\frac{991(990)}{2}$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular
theorem sum_range_991 : ∑ k ∈ range 991, k = 490545 := by sorry
end FiniteTriangular

Source

The closed form for the sum of an initial segment of the nonnegative integers.