チューリング還元
Although there are many known types of oracles, the choice of which to use can be simplified, because many of them are equivalent in computational power. An oracle formula_15 is said to be "polynomially reducible" to another oracle formula_16 if any call to formula_15 may be simulated by an algorithm that accesses the matroid using only oracle formula_16 and takes polynomial time as measured in terms of the number of elements of the matroid; in complexity-theoretic terms, this is a Turing reduction. Two oracles are said to be "polynomially equivalent" if they are polynomially reducible to each other. If formula_15 and formula_16 are polynomially equivalent, then every result that proves the existence or nonexistence of a polynomial time algorithm for a matroid problem using oracle formula_15 also proves the same thing for oracle formula_16.