A reducibility ≤p is a procedure whereby a set A can be computed from a set B. The most general and most extensively studied reducibility is Turing reducibility (≤T). However, when one analyzes effectiveness considerations in classical mathematics, one often discovers that the relevant reducibilities are stronger (i.e., more restrictive) than ≤T. To illustrate, in combinatorial group theory we find that the word problem is many-one reducible to the conjugacy problem, and that word problems occur in each r.e. truth table (tt-) degree (see, for example, Miller [17]).
In the present paper we are concerned with another strong reducibility: weak truth table (wtt-) reducibility. Here the reader should recall that A ≤wtt, β means that there is a procedure Φ and a recursive function φ such that Φ(β) = A and for all x, the u(Φ(β; X)) < φ (x). That is, the amount of information used in the computation is bounded by φ. The critical difference between truth table and weak truth table reducibilities is that for tt we will at once be “given the whole table.” Thus if Δ is a tt-procedure and δ is its use, then for all x and all strings σ of length δ(x) we can figure out Δ(σ; x). On the other hand if Δ is merely a wtt-procedure it may be that for some string σ, Δ(σ; x)↓, whilst for another string μ of the same length it may be that Δ{μ; x) ↑. We remark that wtt-reducibility arises very naturally both in effective algebra and in the structure of the r.e. T-degrees R. The reader should see, for instance, Downey and Remmel [3], where it is shown that the complexity of r.e. bases of an r.e. vector space V is characterised precisely by the wtt-degrees below V, and also Ladner and Sasso [14] or Downey [1], where the wtt-degrees are used to investigate cupping and capping in R.