Sum of integers below 865

Proved

FiniteTriangular.sum_range_865

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

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $865$ equals $373680$ . Equivalently, $\\sum_{k=0}^{865-1} k = 865(865-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_865 : ∑ k ∈ range 865, k = 373680 := 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.