Approximation of unbounded functional, defined by grades of normal operator, on the class of elements of Hilbert space

R.O. Bilichenko
2016 Researches in Mathematics  
We obtain the best approximation of unbounded functional $(A^k x; f)$ on the class $\{ x\in D(A^r) \colon \| A^r x \| \leqslant 1 \}$ by linear bounded functionals for a normal operator $A$ in the Hilbert space $H$ ($k < r$, $f\in H$).
doi:10.15421/241601 fatcat:2zpm4anx5nhntdogqhyayobpl4