Orthogonalization of a system of functions

The construction, for a given system of functions which are square integrable on the segment , of an orthogonal system of functions by using a process of orthogonalization or by extending the functions to a larger interval , .

The use of the Schmidt orthogonalization process for a complete system of functions always reduces it to a complete orthonormal system , and given a corresponding choice of the sequence , permits the construction of a system which possesses some good properties. In this way, for example, the Franklin system (see Orthogonal series) is created, which is a basis in and in , .

Orthogonalization of a system of functions by extension to a larger interval was first introduced by I. Schur (see [1]). He proved that for the existence of a system , , , , orthonormal in , it is necessary and sufficient that the condition

be fulfilled, where the supremum is taken over all with . Necessary and sufficient conditions have also been found such that, when these are fulfilled, one can obtain a complete orthonormal system by means of such an orthogonalization (see [2]).

A number of constructions of orthogonalization by extension of functions are given by D.E. Men'shov [3]. They are used to prove theorems on the accuracy of the condition for the almost-everywhere convergence of an orthogonal series .

References

[1]	S. Kaczmarz, H. Steinhaus, "Theorie der Orthogonalreihen" , Chelsea, reprint (1951)
[2]	A.M. Olevskii, "On the extension of a sequence of functions to a complete orthonormal system" Math. Notes , 6 : 6 (1969) pp. 908–913 Mat. Zametki , 6 : 6 (1969) pp. 737–747
[3]	D.E. Men'shov, "Sur les séries des fonctions orthogonales bornees dans leur ensemble" Mat. Sb. , 3 (1938) pp. 103–120
[4]	Ph. Franklin, "A set of continuous orthogonal functions" Math. Ann. , 100 (1928) pp. 522–529

Comments

The Schmidt orthogonalization process is often called the Gram–Schmidt orthogonalization process.