Bit-level correlations in some pseudorandom number generators.

We present results of extensive bit-level tests on some pseudorandom number generators which are commonly used in physics applications. The generators have first been tested with an extended version of the d-tuple test. Second, we have developed a cluster test where a physical analogy of the binary numbers with the two-dimensional Ising model has been utilized. We demonstrate that this new test is rather powerful in finding periodic correlations on bit level. Results of both test methods are presented for each bit of the output of the generators. Some generators exhibit clear bit-level correlations but we find no evidence of discernible correlations for generators, which have recently produced systematic errors in Monte Carlo simulations.