Sum of the nonnegative integers below 990

Proved

FiniteTriangular.sum_range_990

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

algebrafinset

The sum of the integers from $0$ through $989$ equals $489555$ , which is the triangular number $\\frac{990(989)}{2}$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular
theorem sum_range_990 : ∑ k ∈ range 990, k = 489555 := by sorry
end FiniteTriangular

Source

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