On the Riesz means of expansion in eigenfunctions and associated functions of a nonselfadjoint ordinary differntial operator

For any function $f(x)\in L_1$, the connection between the Riesz means of expansions of $f(x)$ in a trigonometric series and in a biorthogonal series in the eigenfunctions and associated functions of a nonselfadjoint differential operator of order $2n$ is studied.
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