In the first part of this series of papers (see Brudern, Kawada and Wooley [2]), we introduced an approach to additive problems in which one seeks to establish that almost all natural numbers in some fixed polynomial sequence are represented in a prescribed manner, thereby deriving non-trivial estimates for exceptional sets in thin sequences. We illustrated our methods by obtaining upper bounds for the exceptional sets associated with the representation of integers from quadratic, or cubic, polynomial sequences by sums of six cubes of positive integers. In subsequent parts of the series (see Brudern, Kawada and Wooley [3], [4], [5]), we adapted our core methods so as to tackle problems associated with the binary Goldbach problem, the expected asymptotic formula for the number of representations, and lower bounds for the number of integers represented in some prescribed manner. As is apparent from the opening part of this series, our methods are of great flexibility. The aim of the present paper is to provide variants of the ideas developed in the preceding opera, and here we will be concerned solely with methods which provide estimates for the size of exceptional sets in representation problems. The discerning reader will recognise that in several of the more exotic problems mentioned below, it is the existence of a non-trivial estimate for the exceptional set in question which is of interest. The investigation of the sharpest attainable estimate for this exceptional set should be politely deferred beyond any future occasion. We begin by exploring exceptional sets in polynomial sequences for additive problems involving mixed powers. Here one finds that sharp mean value estimates for mixed sums of powers, familiar to aficionados of the circle method, lead to surprisingly strong conclusions. Our first results, which we establish in §2, involve problems containing a block of four cubes. Here and elsewhere, ∗ Packard Fellow, and supported in part by NSF grant DMS-9622773. This paper benefitted from visits of various of the authors to Ann Arbor, Kyoto, Oberwolfach and Stuttgart, and the authors collectively thank these institutions for their hospitality and excellent working conditions. Received August 16, 2000.
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