[[Image:Absolute Tau function for x up to 16,000 with logarithmic scale.JPG|thumbnail|upright=1.64|Values of for n < 16,000 with a logarithmic scale. The blue line picks only the values of n that are multiples of 121.]] The Ramanujan tau function, studied by, is the function defined by the following identity: where with Im z > 0 , \phi is the Euler function, η is the Dedekind eta function, and the function Δ(z) is a holomorphic cusp form of weight 12 and level 1, known as the discriminant modular form (some authors, notably Apostol, write instead of \Delta). It appears in connection to an "error term" involved in counting the number of ways of expressing an integer as a sum of 24 squares. A formula due to Ian G. Macdonald was given in.

Values

The first few values of the tau function are given in the following table : Calculating this function on an odd square number (i.e. a centered octagonal number) yields an odd number, whereas for any other number the function yields an even number.

Ramanujan's conjectures

observed, but did not prove, the following three properties of τ(n) if gcd(m,n) = 1 (meaning that τ(n) is a multiplicative function) τ(pr + 1) = τ(p)τ(pr) − p11 τ(pr − 1) for p prime and r > 0 . for all primes p. The first two properties were proved by and the third one, called the Ramanujan conjecture, was proved by Deligne in 1974 as a consequence of his proof of the Weil conjectures (specifically, he deduced it by applying them to a Kuga-Sato variety).

Congruences for the tau function

For k ∈ \mathbb{Z} and n ∈ \mathbb{Z}>0 , the Divisor function σk(n) is the sum of the kth powers of the divisors of n. The tau function satisfies several congruence relations; many of them can be expressed in terms of σk(n) . Here are some: For p ≠ 23 prime, we have

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Explicit formula

In 1975 Douglas Niebur proved an explicit formula for the Ramanujan tau function: where σ(n) is the sum of the positive divisors of n.

Conjectures on τ(n)

Suppose that f is a weight-k integer newform and the Fourier coefficients a(n) are integers. Consider the problem: a(p) ≢ 0 (mod p) ? Indeed, most primes should have this property, and hence they are called ordinary. Despite the big advances by Deligne and Serre on Galois representations, which determine a(n) (mod p) for n coprime to p, it is unclear how to compute a(p) (mod p) . The only theorem in this regard is Elkies' famous result for modular elliptic curves, which guarantees that there are infinitely many primes p such that , which thus are congruent to 0 modulo p . There are no known examples of non-CM f with weight greater than 2 for which a(p) ≢ 0 (mod p) for infinitely many primes p (although it should be true for almost all p). There are also no known examples with a(p) ≡ 0 (mod p) for infinitely many p. Some researchers had begun to doubt whether a(p) ≡ 0 (mod p) for infinitely many p. As evidence, many provided Ramanujan's τ(p) (case of weight 12). The only solutions up to 1010 to the equation τ(p) ≡ 0 (mod p) are 2, 3, 5, 7, 2411, and 7,758,337,633. conjectured that τ(n) ≠ 0 for all n, an assertion sometimes known as Lehmer's conjecture. Lehmer verified the conjecture for n up to 214,928,639,999 (Apostol 1997, p. 22). The following table summarizes progress on finding successively larger values of N for which this condition holds for all n ≤ N .

Ramanujan's L-function

Ramanujan's L-function is defined by if \Re s>6 and by analytic continuation otherwise. It satisfies the functional equation and has the Euler product Ramanujan conjectured that all nontrivial zeros of L have real part equal to 6.