Finite expression for L(1, χ)

Tob´ıasChavarr´ıaCastro

Universidad de Costa Rica

Abstract

In 1839 the german mathematician Johann Peter Gustav Lejeune Dirichlet proof the prime number theorem in arithmetic progressions, this document provides a introduction to the theory of Dirichlet L-function, and a closed formula to compute the L-function for a particular case, which is important in the proof of the theorem.

Introduction The theory of L-functions, plays a central role in number theory since Dirichlet’s proof his famous theorem, this functions have the special property as the Riemann zeta-function, that their series representation they can also be described by an Euler product, i.e., a product taken over prime numbers. The famous Riemann zeta-function:

∞ X 1 Y ζ(s) = = (1 − p−s)−1 ns n=1 p

Theorem. (L.Dirichlet) Let q ≥ 1 and a ≥ 1 such that (a, q) = 1. Then there are infinity many primes p ≡ a(mod q) and more precisely:

X 1 1 = log log(N) + O(1) p ϕ(q) p≤N, p≡a(modq)

An immediately consequence of this theorem is that in every arithmetic progression:

a, a + q, a + 2q, ... includes infinitely many primes.

1 A “first” proof of this result was only for the case when q is a prime number, that is the case for wich we are going to give a closed formula for L(1, χ). For this proof Dirichlet introduced the arithmetic functions called Dirichlet characters.

Definition 1. Dirichlet characters mod q.

A Dirichlet character modulo q is any function χ from the integers Z to the complex numbers C∗ such that χ has the following properties:

• χ(n + q) = χ(n) for all n ∈ Z

• χ(nk) = χ(n)χ(k) for all n, k ∈ Z

• χ(n) 6= 0 if and only if (n, q) = 1

Notice that this is a completely . In practice the construction of χ can ∗ be done as follows: Let q a prime number, then the multiplicative modulo q, ie (Z/qZ) is cyclic with generator, say g, called a primitive root modulo q, then we pick a complex number χ(g) with χ(g)q−1 = 1 . Then we define χ(gm) = χ(g)m.The simplest character is the trivial character χ0(n) = 1 if (n, q) = 1 and χ0(n) = 0 if (n, q) > 1.

Definition 2. Dirichlet L-Functions Let χ a Dirichlet character modulo q. We define the Dirichlet L-function associated to χ by the formula:

∞ X χ(n) L(s, χ) = ns n=1

An important observation of this definition is the convergence of this series, since |χ(n)| = 1 for all n ∈ Z, we have |χ(n)n−s| <= n−s, this implies that the terms of L(s, χ) are dominated in absolute value by the corresponding terms in ζ(s). Thus L(s, χ) converges and is continuous for s > 1. Moreover since χ is completely multiplicative we have a product formula for L(s, χ) in exactly the same way as for ζ(s). Namely: Y L(s, χ) = (1 − χ(p)p−s)−1 p

Dirichlet considered what can happen to log(L(s, χ)) as s → 1. The character χ0 gives L(1, χ0) = (1 − q−s)ζ(s) and we know that this tends to ∞ as s → 1. The rest of the proof it will suffice to show that, for each character χ other than the trivial, log(L(s, χ)) is bounded as s → 1, the characters are separated into the ones that take complex values (an they come in pairs, with their conjugates), and the real character with χ(g) = −1. Dirichlet could easily consider the complex characters, and see that L(1, χ) 6= 0 and note that that the real one, is much harder to treat. Now we are going to prove that L(1, χ) 6= 0 when χ is a real Dirichlet character modulo q, ie, χ(N) = {1, −1, 0}, in this case the Dirichlet character coincides with the Legendre symbol:

 1 if a is mod q a  ( ) = −1 if a is a quadratic nonresidue mod q q 0 if q|a.

To do this, first we need to define a , and some results on this.

2πi Definition 3. Gauss Sum. Let ζ = e q the Gauss sum of a Dirichlet Character χ is:

q−1 X g(χ) = χ(k)ζk k=1

Lemma 1. The value of the quadratic Gauss sum g(χ) is given by:

(√ q if q ≡ 1(4) g(χ) = √ i q if q ≡ 3(4)

2pii Lemma 2. Let g(χ) the the classical Gauss sum, χ the Legendre symbol, and ζ = e p , q prime. Q N Q R − Define Q = N (1 − ζ ) R(1 − ζ ) 1 Where N runs over all non-quadratic residues modulo p and R runs over all quadratic residues modulo p. Then:

Q = exp(L(1, χ)g(χ)))

Pp−1 a Proof. Let g(χ) be the classical Gauss Sum a=1 χ(a)ζ , where χ is the Legendre symbol and 2πi ζ = e p . Define:

Y Y Q = (1 − ζN ) (1 − ζR)−1 N R Where N runs over all non-quadratic residues modulo p and R runs over all quadratic residues modulo p. P∞ xk Remember that: − log(1 − x) = k=1 k

Y Y log(Q) = log( (1 − ζN ) (1 − ζR)−1) = N R X X = log(1 − ζN ) + − log(1 − ζR) N R ∞ ∞ X X ζNk X X ζRk = − + k k N k=1 R k=1 ∞ X X ζNk X ζRk = ( χ(N) + χ(R) ) k k k=1 N R ∞ ! X χ(k) X X = χ(k)χ(N)ζNk + χ(k)χ(R)ζRk k k=1 N R ∞ ! X χ(k) X X = χ(kN)ζNk + χ(kR)ζRk k k=1 N R ∞ p−1 X χ(k) X = χ(a)ζa k k=1 a=1

P∞ χ(k) Pp−1 a Since L(1, χ) = k=1 k and g(χ) = a=1 χ(a)ζ , we have:

Q = exp(L(1, χ)g(χ)))

Since Q 6= 0 and g(χ) 6= 0, this prove that L(1, χ) is bounded and to prove that L(1, χ) 6= 0 is equivalent to proving that Q 6= 1. For this, Dirichlet had recourse to a result that had been proved by Gauss in his work on cyclotomy. This is that, for an ,indeterminate x, Y 1 √ (x − ζR)= [Y (x) − qW (x)] 2 R and Y 1 √ (x − ζN )= [Y (x) + qW (x)] 2 N

Where Y (x),W (x) ∈ Z[x]. Assuming this we have the identity:

q−1 1 Y [Y 2(x) − qW 2(x)] = (x − ζa) = Φ (x) 4 q a=1

Where Φq(x) denotes the the q-th cyclotomic polynomia, and since q is a prime number we have:

q−1 q−2 Φq(x) = x + x + ... + x + 1

Now putting x = 1, we obtain integers, Y (1) = Y and W (1) = W , which satisfies the Diophantine equation: Y 2 − qW 2 = 4q We note that W 6= 0 since 4q is not a perfect square. Therefore expressing Q in terms of this polynomials we have: √ Y Y Y + W q Q = (1 − ζN ) (1 − ζR)−1 = √ Y − W q N R Since W 6= 0, we have the desired conclusion that Q 6= 1.

We will show by an example the usefulness of this formula.

Example. Consider the non-trivial Dirichlet character modulo 3. First note, A = ( Z )∗ = {1, 2}, since A ∼= A , where A is the set of Dirichlet characters. 3Z A = {χ0, χ} is cyclic group generated by χ, ie, A =< χ >, with:

−→ ∗ −→ ∗ Z C χ (k): Z C χ(k): k ≡ 2(3) −→ −1 0 k −→ 1 k ≡ 1(3) −→ 1 Then χ match with the Legendre symbol,and:

∞ X χ(n) L(1, χ) = = n n=1 ∞ X 1 1 = ( − ) 3k + 1 3k + 2 k=0 ∞ X 1 = (3k + 1)(3k + 2) k=0

This series looks easy to compute, but is not. √ 2πi −1 3 3 Now applying the lemma, with ζ = e = 2 + 2 i, and χ the Legendre symbol modulo 3 we have: Y Y (1 − ζN ) (1 − ζR)−1) = N R = (1 − ζ2)(1 − ζ)−1 πi = (1 + ζ) = e 3 √ And g(χ) = i 3 , since 3 ≡ 3(4)

Therefore: πi √ e 3 = exp(i 3)L(1, χ) and π L(1, χ) = √ 3 3

The importance of this result is because in the proof of the Dirichlet Theorem he needs to prove that this value is not zero, and this is a proof in a special case, moreover is a closed formula to compute this value. In the book Multiplicative Number Theory by Harold Davenport, we found a more specific formula for this value, he split the formula in the case when p ≡ 1(4) and p ≡ 3(4) and gets:

 π − 3 if p ≡ 3(4) p 2 L(1, χ) = log(Q) √  p if p ≡ 1(4) Where: Q sin( πN ) Q = N p Q πR R sin( p ) .