For a half integral weight modular form f we study the signs of the Fourier
coefficients a(n). If f is a Hecke eigenform of level N with real
Nebentypus character, and t is a fixed square-free positive integer with
a(t)î€ =0, we show that for all but finitely many primes p the sequence
(a(tp2m))m​ has infinitely many signs changes. Moreover, we prove
similar (partly conditional) results for arbitrary cusp forms f which are not
necessarily Hecke eigenforms