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4. Y. Sibuya, “Simplification of a system of linear ordinary differential equations about a singular point,” Funkc. Ekvac., No. 4, 29–56 (1962).
5. N. I. Shkil’, I. I. Starun, and V. P. Yakovets’, Asymptotic Integration of the Linear Systems of Differential Equations with Degeneration [in Russian], Vyshcha Shkola, Kiev (1991).