TY JOUR TI ASSET ALLOCATION OPTIMIZATION USING COVARIANCE-BASED PORTFOLIO MODELING KW Markowitz model KW investment portfolio KW optimization KW risk KW return KW diversification KW covariance KW financial mathematics JO MOSCOW ECONOMIC JOURNAL AU Sazonov, A.I. AU Pihtil'kova, O.A. AU Morozova, T.A. AU Borec, A.S. PY 2026 IS 11 PB Electronic science