Sum of integers below 403

Proved

FiniteTriangular.sum_range_403

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

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $403$ equals $81003$ . Equivalently, $\\sum_{k=0}^{403-1} k = 403(403-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_403 : ∑ k ∈ range 403, k = 81003 := 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.