Kaisa Matomaki, Maksym Radziwill, and I have just uploaded to the arXiv our paper “An averaged form of Chowla’s conjecture“. This paper concerns a weaker variant of the famous conjecture of Chowla (discussed for instance in this previous post) that

$\displaystyle \sum_{n \leq X} \lambda(n+h_1) \dots \lambda(n+h_k) = o(X)$

as ${X \rightarrow \infty}$ for any distinct natural numbers ${h_1,\dots,h_k}$, where ${\lambda}$ denotes the Liouville function. (One could also replace the Liouville function here by the Möbius function ${\mu}$ and obtain a morally equivalent conjecture.) This conjecture remains open for any ${k \geq 2}$; for instance the assertion

$\displaystyle \sum_{n \leq X} \lambda(n) \lambda(n+2) = o(X)$

is a variant of the twin prime conjecture (though possibly a tiny bit easier to prove), and is subject to the notorious parity barrier (as discussed in this previous post).

Our main result asserts, roughly speaking, that Chowla’s conjecture can be established unconditionally provided one has non-trivial averaging in the ${h_1,\dots,h_k}$ parameters. More precisely, one has

Theorem 1 (Chowla on the average) Suppose ${H = H(X) \leq X}$ is a quantity that goes to infinity as ${X \rightarrow \infty}$ (but it can go to infinity arbitrarily slowly). Then for any fixed ${k \geq 1}$, we have

$\displaystyle \sum_{h_1,\dots,h_k \leq H} |\sum_{n \leq X} \lambda(n+h_1) \dots \lambda(n+h_k)| = o( H^k X ).$

In fact, we can remove one of the averaging parameters and obtain

$\displaystyle \sum_{h_2,\dots,h_k \leq H} |\sum_{n \leq X} \lambda(n) \lambda(n+h_2) \dots \lambda(n+h_k)| = o( H^{k-1} X ).$

Actually we can make the decay rate a bit more quantitative, gaining about ${\frac{\log\log H}{\log H}}$ over the trivial bound. The key case is ${k=2}$; while the unaveraged Chowla conjecture becomes more difficult as ${k}$ increases, the averaged Chowla conjecture does not increase in difficulty due to the increasing amount of averaging for larger ${k}$, and we end up deducing the higher ${k}$ case of the conjecture from the ${k=2}$ case by an elementary argument.

The proof of the theorem proceeds as follows. By exploiting the Fourier-analytic identity

$\displaystyle \int_{{\mathbf T}} (\int_{\mathbf R} |\sum_{x \leq n \leq x+H} f(n) e(\alpha n)|^2 dx)^2\ d\alpha$

$\displaystyle = \sum_{|h| \leq H} (H-|h|)^2 |\sum_n f(n) \overline{f}(n+h)|^2$

(related to a standard Fourier-analytic identity for the Gowers ${U^2}$ norm) it turns out that the ${k=2}$ case of the above theorem can basically be derived from an estimate of the form

$\displaystyle \int_0^X |\sum_{x \leq n \leq x+H} \lambda(n) e(\alpha n)|\ dx = o( H X )$

uniformly for all ${\alpha \in {\mathbf T}}$. For “major arc” ${\alpha}$, close to a rational ${a/q}$ for small ${q}$, we can establish this bound from a generalisation of a recent result of Matomaki and Radziwill (discussed in this previous post) on averages of multiplicative functions in short intervals. For “minor arc” ${\alpha}$, we can proceed instead from an argument of Katai and Bourgain-Sarnak-Ziegler (discussed in this previous post).

The argument also extends to other bounded multiplicative functions than the Liouville function. Chowla’s conjecture was generalised by Elliott, who roughly speaking conjectured that the ${k}$ copies of ${\lambda}$ in Chowla’s conjecture could be replaced by arbitrary bounded multiplicative functions ${g_1,\dots,g_k}$ as long as these functions were far from a twisted Dirichlet character ${n \mapsto \chi(n) n^{it}}$ in the sense that

$\displaystyle \sum_p \frac{1 - \hbox{Re} g(p) \overline{\chi(p) p^{it}}}{p} = +\infty. \ \ \ \ \ (1)$

(This type of distance is incidentally now a fundamental notion in the Granville-Soundararajan “pretentious” approach to multiplicative number theory.) During our work on this project, we found that Elliott’s conjecture is not quite true as stated due to a technicality: one can cook up a bounded multiplicative function ${g}$ which behaves like ${n^{it_j}}$ on scales ${n \sim N_j}$ for some ${N_j}$ going to infinity and some slowly varying ${t_j}$, and such a function will be far from any fixed Dirichlet character whilst still having many large correlations (e.g. the pair correlations ${\sum_{n \leq N_j} g(n+1) \overline{g(n)}}$ will be large). In our paper we propose a technical “fix” to Elliott’s conjecture (replacing (1) by a truncated variant), and show that this repaired version of Elliott’s conjecture is true on the average in much the same way that Chowla’s conjecture is. (If one restricts attention to real-valued multiplicative functions, then this technical issue does not show up, basically because one can assume without loss of generality that ${t=0}$ in this case; we discuss this fact in an appendix to the paper.)