On the computation of coefficients of modular forms: the reduction   modulo p approach
  In this paper we present a probabilistic algorithm to compute the coefficients of modular forms of level one. Focus on the Ramanujan's tau function, we give out the explicit complexity of the algorithm. From a practical viewpoint, the algorithm is particularly well suited for implementations. 
Jinxiang Zeng and Linsheng Yin
math.NT