A log-free zero-density estimate and small gaps in coefficients of $L$-functions

Let $L(s, π\timesπ^\prime)$ be the Rankin--Selberg $L$-function attached to automorphic representations $π$ and $π^\prime$. Let $\tildeπ$ and $\tildeπ^\prime$ denote the contragredient representations associated to $π$ and $π^\prime$. Under the assumption of certain upper bounds for coefficients of the logarithmic derivatives of $L(s, π\times\tildeπ)$ and $L(s, π^\prime\times\tildeπ^\prime)$, we prove a log-free zero-density estimate for $L(s, π\timesπ^\prime)$ which generalises a result due to Fogels in the context of Dirichlet $L$-functions. We then employ this log-free estimate in studying the distribution of the Fourier coefficients of an automorphic representation $π$. As an application we examine the non-lacunarity of the Fourier coefficients $b_f(p)$ of a modular newform $f(z)=\sum_{n=1}^{\infty} b_f(n) e^{2πi n z}$ of weight $k$, level $N$, and character $χ$. More precisely for $f(z)$ and a prime $p$, set $j_f(p):=\max_{x;~x> p} J_{f} (p, x)$, where $J_{f} (p, x):=\#\{{\rm prime}~q;~a_π(q)=0~{\rm for~all~}p<q\leq x\}.$ We prove that $j_f(p)\ll_{f, θ} p^θ$ for some $0<θ<1$.

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