1. Bolibruch A.A. The Riemann-Hilbert problem on a complex projective line. Mat. Zametki (Math. Notes), v.46, No.3, p.118–120 (1989) (in Russian).
2. Bolibruch A.A. The Riemann-Hilbert problem. Uspekhi Mat. Nauk (Soviet mathematical surveys), v.45, No.2, p.3–47 (1990) (in Russian).
3. Anosov D.V. Hilbert's 21st problem (according to Bolibruch). IMA preprint series, #660 Inst. for Math. and its appl. Univ. of Minnesota (1990).
4. Röhrl H. Das Riemann-Hilbertsche Problem der Theorie der linearen Differentialgleichungen. Math. Ann., b.133, s.1–25 (1957).
5. Förster O. Riemannsche Flächen. Berlin e.a., Springer-Verlag, 1977.