lean_workbook_plus_31422

⚠️ 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.

We know that a year is a leap year if the year is divisible by $4$, unless it is divisible by $100$. However, years divisible by $400$ are leap years.

Why this node was retired

The posted statement is

theorem lean_workbook_plus_31422 : ∀ y : ℕ, (y % 4 = 0 ∧ y % 100 ≠ 0) ∨ (y % 400 = 0) ↔ (y % 4 = 0)   :=  by sorry

The correct Gregorian leap-year predicate was equated to divisibility by4 alone.

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

Proposed corrected statement

Define leap(y) iff(4∣y and100∤y) or400∣y; do not simplify it to4∣y. In particular100 is not a leap year but400 is.

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.