1. T.M. Cherry,Analytic quasi-periodic curves of discontinuous type on a torus, Proc. London Math. Soc. v. 44 (1938), 175–215.
2. A. Denjoy,Sur les courbes définies par les équations differentielles à la surface du tore, J. Math. Pures Appl (9) 11 (1932) pp. 333–375.
3. C. Gutierrez,Smoothability of Cherry flows on two-manifolds, To appear.
4. C. Gutierrez,Smooth nonorientable nontrivial recurrence on two-manifolds, To appear in J. Differential Equations.
5. C. Gutierrez,Structural stability for flows on the torus with a cross-cap, Trans. Amer. Math. Soc. 241 (1978), 311–320.