λ(n) = (−1)^Ω(n) where Ω(n) counts prime factors with multiplicity. The Pólya conjecture (1919) claimed L(x) = Σλ(n) ≤ 0 for all x ≥ 2. Disproved by Haselgrove (1958) — the first counterexample is around x ≈ 906,150,257.