Improved bounds for five-term arithmetic progressions

Abstract: Let $r_5(N)$ be the largest cardinality of a set in ${1,\ldots,N}$ which does not contain $5$ elements in arithmetic progression. Then there exists a constant $c\in (0,1)$ such that [r_5(N)\ll \frac{N}{\exp((\log\log N)^{c})}.] Our work is a consequence of recent improved bounds on the $U^4$-inverse theorem of the first author and the fact that $3$-step nilsequences may be approximated by locally cubic functions on shifted Bohr sets. This combined with the density increment strategy of Heath-Brown and Szemer{\'e}di, codified by Green and Tao, gives the desired result.