1. A. Yu. Aleksandrov and A. V. Platonov, Comparison Method and Stability of the Motion of Nonlinear Systems [in Russian], Izd. SPb Univ., St.-Petersburg (2012).
2. Yu. D. Burago and V. A. Zalgaller, Geometric Inequalities, Springer Verlag, Berlin (1988).
3. A. M. Lyapunov, The General Problem of the Stability of Motion, Taylor & Francis, London (1992).
4. A. A. Martynyuk, “Comparison principle based on Minkowski mixed volumes for a family of differential equations,” Diff. Eqs., 53, No. 12, 1549–1556 (2017).
5. K. Conrad, Zorn’s Lemma and Some Applications. Expository papers (2016).