Truth table reduction

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In computability theory, a truth table reduction is a reduction from one set of natural numbers to another. Truth table reductions are less powerful than Turing reductions, since not every Turing reduction between sets can be performed by a truth table reduction, but every truth table reduction can be performed by a Turing reduction. A weak truth table reduction is a related type of reduction which is so named because it weakens the constraints placed on a truth table reduction, and provides a weaker equivalence classification; as such, a "weak truth table reduction" can actually be more powerful than a truth table reduction, and perform a reduction which is not performable by truth table.

A Turing reduction from a set B to a set A computes the membership of a single element in A by asking questions about the membership of various elements in B during the computation; it may adaptively determine which questions it asks based upon answers to previous questions. In contrast, a truth table reduction or a weak truth table reduction must present all of its (finitely many) oracle queries at the same time. In a truth table reduction, the reduction also gives a boolean function (a truth table) which, when given the answers to the queries, will produce the final answer of the reduction. In a weak truth table reduction, the reduction uses the oracle answers as a basis for further computation which may depend on the given answers but may not ask further questions of the oracle.

Equivalently, a weak truth table reduction is a Turing reduction for which the use of the reduction is bounded by a computable function. For this reason, they are sometimes referred to as bounded Turing (bT) reductions rather than as weak truth table (wtt) reductions.

[edit] Properties

As every truth table reduction is a Turing reduction, if A is truth table reducible to B (A ≤_tt B), then A is also Turing reducible to B (A ≤_T B). Considering also many-one reducibility and weak truth table reducibility, one gets the following chain of implications:

• ; many-one reducibility implies truth table reducibility, which in turn implies weak truth table reducibility, which in turn implies Turing reducibility.