# Markov criterion

for best integral approximation

A theorem which in some cases enables one to give effectively the polynomial and the error of best integral approximation of a function . It was established by A.A. Markov in 1898 (see [1]). Let , , be a system of linearly independent functions continuous on the interval , and let the continuous function change sign at the points in and be such that

If the polynomial

has the property that the difference changes sign at the points , and only at those points, then is the polynomial of best integral approximation to and

For the system on , can be taken to be ; for the system , , can be taken to be ; and for the system , , one can take .

#### References

 [1] A.A. Markov, "Selected works" , Moscow-Leningrad (1948) (In Russian) [2] N.I. [N.I. Akhiezer] Achiezer, "Theory of approximation" , F. Ungar (1956) (Translated from Russian) [3] I.K. Daugavet, "Introduction to the theory of approximation of functions" , Leningrad (1977) (In Russian)