Define a bit-guessing algorithm to be any algorithm that, given a finite string of bits as input, predict what the next bit will be; define the bit-guessing game as an adversarial game in which Player A chooses a bit-guessing algorithm, Player B chooses an infinite sequence of bits, and the score is the asymptotic density (maybe I should say upper density or lower density?) of the set of n’s for which A’s algorithm correctly predicts B’s nth bit. Question: If Player B is given complete knowledge of the innards of a million bit-guessing algorithms, can B construct an infinite sequence of bits whose score lies between .49 and .51 for *all* of those algorithms? Of course by “a million” I mean “n”, and by .49 and .51 I mean one-half plus epsilon and one-half minus epsilon, with suitable quantifiers. I want a (nonrandom) construction, not an existence proof. We’ll allow B to consult an oracle that will answer questions about those million bit-guessing algorithms (e.g., the oracle will reveal the truth-status of the Riemann Hypothesis if that’s germane). This is probably an old problem, but I’m not sure how to search for it. Jim Propp