# Korovkin theorems

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Korovkin's first theorem states that if is an arbitrary sequence of positive linear operators on the space of real-valued continuous functions on the interval (cf. Continuous functions, space of; Linear operator) and if for all , where ( , ), then for all .

The statement of Korovkin's second theorem is similar to that of the first theorem, but is replaced by (the space of -periodic real-valued functions on , endowed with the topology of uniform convergence on ) and is taken from the set .

These theorems were proved by P.P. Korovkin in 1953 ([a3], [a4]). In 1952, H. Bohman [a2] had proved a result similar to Korovkin's first theorem but concerning sequences of positive linear operators on of the form where is a finite set of numbers in and ( ). Therefore Korovkin's first theorem is also known as the Bohman–Korovkin theorem. However, T. Popoviciu [a5] had already proved the essence of the theorem in 1950.

Korovkin has tried to generalize his first theorem by replacing with other finite subsets of . He has shown that if a subset "behaves like" , then (Korovkin's third theorem). Moreover, he showed that a subset "behaves like" if and only if it is a Chebyshev system of order two.

The Korovkin theorems are simple yet powerful tools for deciding whether a given sequence of positive linear operators on or is an approximation process. Furthermore, they have been the source of a considerable amount of research in several other fields of mathematics (cf. Korovkin-type approximation theory).

See [a1] for a modern and comprehensive exposition of these results and for (some) applications.

How to Cite This Entry:
Korovkin theorems. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Korovkin_theorems&oldid=13016
This article was adapted from an original article by F. Altomare (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article