Glasgow Math. J. 53 (2011) 1–50. C Glasgow Mathematical Journal Trust 2010. doi:10.1017/S0017089510000546. AN INVERSE THEOREM FOR THE GOWERS U4-NORM BEN GREEN Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK e-mail:
[email protected] TERENCE TAO UCLA Department of Mathematics, Los Angeles, CA 90095-1555, USA e-mail:
[email protected] and TAMAR ZIEGLER Department of Mathematics, Technion - Israel Institute of Technology, Haifa 32000, Israel e-mail:
[email protected] (Received 30 November 2009; accepted 18 June 2010; first published online 25 August 2010) Abstract. We prove the so-called inverse conjecture for the Gowers U s+1-norm in the case s = 3 (the cases s < 3 being established in previous literature). That is, we show that if f :[N] → ރ is a function with |f (n)| 1 for all n and f U4 δ then there is a bounded complexity 3-step nilsequence F(g(n)) which correlates with f . The approach seems to generalise so as to prove the inverse conjecture for s 4as well, and a longer paper will follow concerning this. By combining the main result of the present paper with several previous results of the first two authors one obtains the generalised Hardy–Littlewood prime-tuples conjecture for any linear system of complexity at most 3. In particular, we have an asymptotic for the number of 5-term arithmetic progressions p1 < p2 < p3 < p4 < p5 N of primes. 2010 Mathematics Subject Classification. Primary: 11B30; Secondary: 37A17. NOTATION . By a 1-bounded function on a set X we mean a function f : X → ރ with |f (x)| 1 for all x∈ X.