Divergence of weighted square averages in $L^1$ (1907.11060v3)

Abstract: We study convergence of ergodic averages along squares with polynomial weights. For a given polynomial $P\in \mathbb{Z}[\cdot]$, consider the set of all $\theta\in[0,1)$ such that for every aperiodic system $(X,\mu, T)$ there is a function $f\in L^1(X,\mu)$ such that the weighted averages along squares $$ {\frac{1}{N}\sum_{n=1}^N} e(P(n)\theta)T^{n^2}f $$ diverge on a set with positive measure. We show that this set is residual and includes the rational numbers as well as a dense set of Liouville numbers. This on one hand extends the divergence result for squares in $L^1$ of the first author and Mauldin and on the other hand shows that the convergence result for linear weights for squares due to the second author and Krause in $L^p$, $p>1$ does not hold for $p=1$.