Function studied by Ramanujan
The Ramanujan tau function, studied by Ramanujan (1916), is the function formula_1 defined by the following identity:
formula_2
where "q" 
 exp(2"πiz") with Im "z" ; 0, formula_3 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 formula_4 instead of formula_5). 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 (sequence in the OEIS):
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"):
 "τ"("m")"τ"("n") if gcd("m","n") 
 1 (meaning that "τ"("n") is a multiplicative function)
 "τ"("p")"τ"("pr") − "p"11 "τ"("p""r" − 1) for p prime and "r" ; 0.
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" ∈ formula_6 and "n" ∈ formula_6 ;0, the Divisor function "σ""k"("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 "σ""k"("n"). Here are some:
For "p" ≠ 23 prime, we have
Explicit formula.
In 1975 Douglas Niebur proved an explicit formula for the Ramanujan tau function:
formula_18
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: 
 Given that f does not have complex multiplication, do almost all primes p have the property that "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 "a"("p") 
 0, 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 (sequence in the OEIS).
 conjectured that "τ"("n") ≠ 0 for all n, an assertion sometimes known as Lehmer's conjecture. Lehmer verified the conjecture for n up to (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
formula_19
if formula_20 and by analytic continuation otherwise. It satisfies the functional equation
formula_21
and has the Euler product
formula_22
Ramanujan conjectured that all nontrivial zeros of formula_23 have real part equal to formula_24.
Notes.
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