Semicomputable function

that can be approximated either from above or from below by a computable function.

More precisely a partial function

is upper semicomputable, meaning it can be approximated from above, if there exists a computable function

is the level of approximation, such that: Completely analogous a partial function

is upper semicomputable or equivalently if there exists a computable function

such that: If a partial function is both upper and lower semicomputable it is called computable.

This mathematical logic-related article is a stub.