STRYGIN, A. The principle of averaging for multi-dimentional differential equations of the first order. Lithuanian Mathematical Journal, [S. l.], v. 7, n. 4, p. 673–677, 1967. DOI: 10.15388/LMJ.1967.20016. Disponível em: https://www.journals.vu.lt/LMJ/article/view/20016.. Acesso em: 8 may. 2024.