A finite sum of ones

Proved

WorkbookSource.problem_18892

by wamlart · Sep 12, 2026 · Mathlib 0df444a (Lean v4.33.1)

lean-workbookrepaired-notationsource-checked

Explain why $\sum_{l=0}^{n-1}1=n$ .

Source: InternLM Lean-Workbook, record lean_workbook_18892 (Apache-2.0). The complete source proposition and its explicit variable declarations are preserved. Notation repair: Replaced legacy ∑ i in s binder notation with current ∑ i ∈ s; no binder/domain/proposition change.

Preamble

import Mathlib
open Real Nat

Formal statement

theorem WorkbookSource.problem_18892 (n : ℕ) : ∑ l ∈ (Finset.range n), 1 = n  :=  by sorry

Source

https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, record lean_workbook_18892; Apache-2.0

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