Optimal bounds for sums of bounded arithmetic functions

Let $A(s) = \sum_n a_n n^{-s}$ be a Dirichlet series with meromorphic continuation. Say we are given information on the poles of $A(s)$ with $|\Im s| \leq T$ for some large constant $T$. What is the best way to use such finite spectral data to give explicit estimates on sums $\sum_{n\leq x} a_n$? The problem of giving explicit bounds on the Mertens function $M(x) = \sum_{n\leq x} \mu(n)$ illustrates how open this basic question was. Bounding $M(x)$ might seem equivalent to estimating $\psi(x) = \sum_{n\leq x} \Lambda(n)$ or the number of primes $\leq x$. However, we have long had fairly good explicit bounds on prime counts, while bounding $M(x)$ remained a notoriously stubborn problem. We prove a sharp, general result on sums $\sum_{n\leq x} a_n n^{-\sigma}$ for $a_n$ bounded, giving an optimal way to use information on the poles of $A(s)$ with $|\Im s|\leq T$ and no data on the poles above. Our bounds on $M(x)$ are stronger than previous ones by many orders of magnitude. (Similar results for $\psi(x)$ are given in a companion paper.) Using rigorous residue computations by D. Platt, we obtain, for $x\geq 1$, $$|M(x)|\leq \frac{3}{\pi\cdot 10^{10}}\cdot x + 11.39 \sqrt{x}.$$ This is a corollary of our main result, essentially an explicit formula with the contribution of each pole clearly stated; we shall discuss how this finer structure can be useful. Our proof mixes a Fourier-analytic approach in the style of Wiener--Ikehara with contour-shifting, using optimal approximants of Beurling--Selberg type (Carneiro--Littmann, 2013); for $\sigma=1$, the approximant in (Vaaler, 1985) reappears. While we proceed independently of existing explicit work on $M(x)$ and $\psi(x)$, our method has an important step in common with work on another problem by (Ramana--Ramar\'e, 2020).