τ(n) are the Fourier coefficients of Δ(q) = q∏(1-qⁿ)²⁴ = Σ τ(n)qⁿ, the unique weight-12 cusp form. Ramanujan conjectured (Deligne proved 1974): |τ(p)| ≤ 2p^(11/2) for primes p.
τ(1)=1, τ(2)=−24, τ(3)=252, τ(4)=−1472, τ(5)=4830 — multiplicative: τ(mn)=τ(m)τ(n) when gcd(m,n)=1