In mathematics, the Feferman-Schütte ordinal ?0 is a large countable ordinal. It is the proof-theoretic ordinal of several mathematical theories, such as arithmetical transfinite recursion. It is named after Solomon Feferman and Kurt Schütte.
It is sometimes said to be the first impredicative ordinal, though this is controversial, partly because there is no generally accepted precise definition of "predicative". Sometimes an ordinal is said to be predicative if it is less than ?0.
There is no standard notation for ordinals beyond the Feferman-Schütte ordinal. There are several ways of representing the Feferman-Schütte ordinal, some of which use ordinal collapsing functions: , or .
The Feferman-Schütte ordinal can be defined as the smallest ordinal that cannot be obtained by starting with 0 and using the operations of ordinal addition and the Veblen functions ??(?). That is, it is the smallest ? such that ??(0) = ?.