Note to the existence of periodic solutions for higher-order differential equations with nonlinear restoring term and time-variable coefficients Ján Andres, Vladimír Vlček (1991) Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica
Note to the Lagrange stability of excited pendulum type equations Ján Andres, Staněk, Svatoslav (1993) Mathematica Slovaca
Note to the paper of Fučík and Mawhin Ján Andres (1990) Commentationes Mathematicae Universitatis Carolinae