Stability in the large of systems of two equations

Barbashin, E. A., & N. N. Krasovskii: On the stability of motion in the large. Dokl. Akad. Nauk SSSR. (N.S.) 86, 453–456 (1952). [Russian.]

Ershov, B. A.: On the stability in the large of a certain system of automatic regulation. Prikl. Mat. Meh. 17, 61–72 (1953). [Russian.]

Erugin, N. P.: On certain questions about the stability of motion and the qualitative theory of differential equations in the large. Prikl. Mat. Meh. 14, 459–512 (1950). [Russian.]

Frommer, M.: Die Integralkurven einer gewöhnlichen Differentialgleichung erster Ordnung in der Umgebung rationaler Unbestimmtheitsstellen. Math. Ann. 99, 222–272 (1928).