Maths Olympiad Prep

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Number theory Difficulty 6.1 National olympiad Prove it

24. Let χ\chi be a non-principal character modulo kk. Prove:
(i) When s>0s>0, the series n=1χ(n)ns\sum_{n=1}^{\infty} \chi(n) n^{-s} converges, and when s>1s>1,
L(s,χ)=n=1χ(n)ns=p(1χ(p)ps)1L(s, \chi)=\sum_{n=1}^{\infty} \chi(n) n^{-s}=\prod_{p}\left(1-\frac{\chi(p)}{p^{s}}\right)^{-1}
(ii) When s>1s>1,
L1(s,χ)=n=1μ(n)χ(n)nsL^{-1}(s, \chi)=\sum_{n=1}^{\infty} \mu(n) \chi(n) n^{-s}
(iii) When s>1s>1,
lnL(s,χ)=n=2Λ(n)χ(n)(lnn)1ns;\ln L(s, \chi)=\sum_{n=2}^{\infty} \Lambda(n) \chi(n)(\ln n)^{-1} n^{-s} ;
(iv) When s>1s>1,
L(s,χ)L(s,χ)=n=1Λ(n)χ(n)ns-\frac{L^{\prime}(s, \chi)}{L(s, \chi)}=\sum_{n=1}^{\infty} \Lambda(n) \chi(n) n^{-s}

Solution

24. (i) The first part uses Question 21, the second part is a special case of Question 2 of Exercise Three in Chapter 8;
(ii) Prove similarly to Question 3 of Exercise Three in Chapter 8; (iii) Use (i); (iv) Use (i) or (iii).

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.