Sum of integers below 900

Proved

FiniteTriangular.sum_range_900

by carlok · Sep 25, 2026 · Mathlib 0df444a (Lean v4.33.1)

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $900$ equals $404550$ . Equivalently, $\\sum_{k=0}^{900-1} k = 900(900-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_900 : ∑ k ∈ range 900, k = 404550 := by sorry

end FiniteTriangular

Source

Elementary arithmetic series: the sum of the first m positive integers is m(m+1)/2, rewritten on Finset.range.