Counting primes in arithmetic progressions

Abstract

These notes represent an expanded version of a lecture delivered at the Urbana meeting of June 2014 in memory of Paul and Felice Bateman and of Heini Halberstam, and, in modified form, at the October 2014 workshop at the Royal Swedish Academy of Sciences, Stockholm, on the occasion of the presentation to Yitang Zhang of the 2014 Rolf Schock Prize inMathematics for his ground-breaking work on bounded gaps between primes.