Roth’s Orthogonal Function Method in Discrepancy Theory and Some New Connections Dmitriy Bilyk Abstract In this survey we give a comprehensive, but gentle introduction to the cir- cle of questions surrounding the classical problems of discrepancy theory, unified by the same approach originated in the work of Klaus Roth [85] and based on mul- tiparameter Haar (or other orthogonal) function expansions. Traditionally, the most important estimates of the discrepancy function were obtained using variations of this method. However, despite a large amount of work in this direction, the most im- portant questions in the subject remain wide open, even at the level of conjectures. The area, as well as the method, has enjoyed an outburst of activity due to the recent breakthrough improvement of the higher-dimensional discrepancy bounds and the revealed important connections between this subject and harmonic analysis, proba- bility (small deviation of the Brownian motion), and approximation theory (metric entropy of spaces with mixed smoothness). Without assuming any prior knowledge of the subject, we present the history and different manifestations of the method, its applications to related problems in various fields, and a detailed and intuitive outline of the latest higher-dimensional discrepancy estimate. 1 Introduction The subject and the structure of the present chapter is slightly unconventional. In- stead of building the exposition around the results from one area, united by a com- mon topic, we concentrate on problems from different fields which all share a com- mon method. The starting point of our discussion is one of the earliest results in discrepancy theory, Roth’s 1954 L2 bound of the discrepancy function in dimensions d ≥ 2 [85], as well as the tool employed to obtain this bound, which later evolved into a pow- Dmitriy Bilyk Department of Mathematics, University of South Carolina, Columbia, SC 29208 USA e-mail:
[email protected] 1 2 Dmitriy Bilyk erful orthogonal function method in discrepancy theory.