Determination of whether predicate is true or false for any given values of , ..., is called its decision problem. The decision problem for predicate is called recursively decidable if there is a total recursive function such that
Given the equivalence of computability and recursiveness, this definition may be restated with reference to computable functions instead of recursive functions.
The halting problem was one of the first to be shown recursively undecidable. The formulation of recursive undecidability of the halting problem and many other recursively undecidable problems is based on Gödel numbers. For instance, the problem of deciding for any given whether the Turing machine whose Gödel number is is total is recursively undecidable. Hilbert's tenth problem is perhaps the most famous recursively undecidable problem.
Most proofs of recursive undecidability use reduction. They show that recursive decidability of the problem under study would imply recursive decidability of another problem known to be recursively undecidable. As far as direct proofs are concerned, they usually employ the idea of the Cantor diagonal method.