Simulating BPP using a general weak random source

It is shown how to simulate BPP and approximation algorithms in polynomial time using the output from a δ-source. A δ-source is a weak random source that is asked only once for R bits, and must output an R-bit string according to some distribution that places probability no more than 2^-δR on any particular string. Also given are two applications: one to show the difficulty of approximating the size of the maximum clique, and the other to the problem of implicit O(1) probe search