On a cosine operator function framework of approximation processes in Banach space

Andi Kivinukk, Anna Saksa, Maria Zeltser


We introduce the cosine-type approximation processes in abstract Banach space setting.
The historical roots of these processes go back to W. W. Rogosinski in 1926. The given new denitions
use a cosine operator functions concept. We proved that in presented setting the cosine-type operators
possess the order of approximation, which coincide with results known in trigonometric approximation.
Moreover, a general method for factorization of certain linear combinations of cosine operator functions
are presented. The given method allows to nd the order of approximation using the higher order
modulus of continuity. Also applications for the dierent type of approximations are given.


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