Small values of $| L^\prime/L(1,\chi) |$

Abstract

In this paper, we investigate the quantity $m_q:=\min_{\chi\ne \chi_0} | L^\prime/L(1,\chi)|$, as $q\to \infty$ over the primes, where $L(s,\chi)$ is the Dirichlet $L$-function attached to a non trivial Dirichlet character modulo $q$. Our main result shows that $m_q \ll \log\log q/\sqrt{\log q}$. We also compute $m_q$ for every odd prime $q$ up to $10^7$. As a consequence we numerically verified that for every odd prime $q$, $3 \le q \le 10^7$, we have $c_1/q< m_q<5/\sqrt{q}$, with $c_1=21/200$. In particular, this shows that $L^\prime(1,\chi) \ne 0$ for every non trivial Dirichlet character $\chi$ mod $q$ where $3\leq q\leq 10^7$ is prime, answering a question of Gun, Murty and Rath in this range. We also provide some statistics and scatter plots regarding the $m_q$-values, see Section 6. The programs used and the computational results described here are available at the following web address: \url{http://www.math.unipd.it/~languasc/smallvalues.html}.