On the distribution of modular inverses from short intervals

Abstract

For a prime number $p$ and integer $x$ with $\gcd(x,p)=1$ let $\overline{x}$ denote the multiplicative inverse of $x$ modulo $p.$ In the present paper we are interested in the problem of distribution modulo $p$ of the sequence $$ \overline{x}, \qquad x =1, \ldots, N, $$ and in lower bound estimates for the corresponding exponential sums. As representative examples, we state the following two consequences of the main results. For any fixed $A > 1$ and for any sufficiently large integer $N$ there exists a prime number $p$ with $$ (\log p)^A \asymp N $$ such that $$ \max_{\gcd(a,p)=1}\left|\sum_{x\le N}{\mathbf e}_p(a\overline{x})\right|\gg N. $$ For any fixed positive $\gamma< 1$ there exists a positive constant $c$ such that the following holds: for any sufficiently large integer $N$ there is a prime number $p > N$ such that $$ N > \exp\left(c(\log p\log\log p)^{\gamma/(1+\gamma)}\right) $$ and $$ \max_{\gcd(a,p)=1} \left|\sum_{x\le N}{\mathbf e}_p(a \overline{x})\right|\gg N^{1-\gamma}. $$