lean_workbook_plus_41960

⚠️ Retired — specification defect

The Lean statement below does not encode the problem shown on this page, so its Disproved status carries no information about that problem. Do not import this node or use it as a dependency.

Let $f(n)$ be the number of ways to climb $n$ stairs. We have $f(1) = f(2) = 1$ and $f(3) = 2$ . Also for $n \ge 4$ we have $f(n) = f(n-1) + f(n-3)$ since the frog's last step is either a 1-step jump or a 3-step jump. Then just evaluate $f(9)$ .

Why this node was retired

The posted statement is

theorem lean_workbook_plus_41960 (f : ℕ → ℕ) (hf : f 1 = 1 ∧ f 2 = 1 ∧ f 3 = 2 ∧ ∀ n ≥ 4, f n = f (n-1) + f (n-3)) : f 9 = 24   :=  by sorry

Replace the invented value f(9)=24 by f(9)=19 under the stated recurrence.

The recorded counterexample refutes the statement as encoded. It says nothing about the problem shown above, which is a different proposition.

Proposed corrected statement

Replace the invented value f(9)=24 by f(9)=19 under the stated recurrence.

Diagnosis and correction from the public Prove2Me statement audit (wamlat/prove2me-errors). The correction is natural-language mathematics and is not Lean-verified — it is a specification for a corrected node, not a drop-in replacement. No corrected replacement node exists yet.