*The original file was a draft of a letter of reference, later it was edited and renamed.*

**No longer updated after 1997.**

## Work at State Optical Institute under supervision of A. I. Sherstyuk |

From 1980, A. Sergeev is a co-worker in A. I. Sherstyuk research group at State Optical Institute working in the field of atomic physics. A. I. Sherstyuk guided A. S. Kandidat Thesis (1986).

At the beginning of their cooperation A. S. quickly joined in atomic calculations based on Sturmian expansions. Soon, A. S. displayed initiative in developing new methods in perturbation theory and summation of corresponding series. In this area, A. S. became a self-dependent research worker. Publications of A. S. are devoted to efficient theoretical methods in atomic and nuclear physics. The results may be used in high-precision atomic calculations.

A. S. and A. I. Sherstyuk have several joint publications on high-order perturbation theory for screened Coulomb potential and Pade summation of divergent perturbation series. A. S. has derived recursion relations for perturbation coefficients suitable for computer realization. Also, he developed new summation technique to obtain complex eigenvalue from real perturbation series in the case of quasistationary states.

The results of A. S. research may be used by theoretical physicists specialized in atomic theory and quantum-mechanical perturbation theory. He will probably have further contributions in the fields where original mathematical ideas are needed.

See also Unpublished Report-95 for State Optical Institute (in Russian).

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