Turing reduction
As each of the equivalent definitions of a Martin-Löf random sequence is based on what is computable by some Turing machine, one can naturally ask what is computable by a Turing oracle machine. For a fixed oracle "A", a sequence "B" which is not only random but in fact, satisfies the equivalent definitions for computability relative to "A" (e.g., no martingale which is constructive relative to the oracle "A" succeeds on "B") is said to be random relative to "A". Two sequences, while themselves random, may contain very similar information, and therefore neither will be random relative to the other. Any time there is a Turing reduction from one sequence to another, the second sequence cannot be random relative to the first, just as computable sequences are themselves nonrandom; in particular, this means that Chaitin's Ω is not random relative to the halting problem.