A new proof of Halasz's theorem, and its consequences

Halasz's theorem gives an upper bound for the mean value of a multiplicative function f. The bound is sharp for general such f, and, in particular, it implies that a multiplicative function with vertical bar f(n)vertical bar <= 1 has either mean value 0, or is 'close to' n(it) for some fixed t. The proofs in the current literature have certain features that are difficult to motivate and which are not particularly flexible. In this article we supply a different, more flexible, proof, which indicates how one might obtain asymptotics, and can be modified to treat short intervals and arithmetic progressions. We use these results to obtain new, arguably simpler, proofs that there are always primes in short intervals (Hoheisel's theorem), and that there are always primes near to the start of an arithmetic progression (Linnik's theorem).