A prime plus a square: A conditional 11/20 upper bound

Source abstract

Let $E_\square(N)$ count the positive integers at most $N$ that cannot be expressed as a prime plus a nonnegative integer square. Assuming that every nontrivial zero of every primitive Dirichlet $L$-function has real part at most $7/8$, we prove $E_\square(N)\ll_\eps N^{11/20+\eps}$ for every $\eps>0$. The proof combines a conductor-weighted mean-square estimate with a uniform estimate for certain families having a prescribed quadratic component. For these families, the large sieve depends on the varying part of the conductor, while an approximate functional equation controls the fourth moment using the full conductor. A zero-detection argument then absorbs all bands of zeros with real part at least $129/167$. The remaining bands and the minor arcs give the exponent $11/20$. The proof includes even denominators, completion and positivity of the singular series, and removal of proper prime powers.