We study four distinct second-order nonlinear equations of Rabelo which
describe pseudospherical surfaces. By transforming these equations to the
constant-characteristic form we relate them to some well-studied integrable
equations. Two of the Rabelo equations are found to be related to the
sine-Gordon equation. The other two are transformed into a linear equation and
the Liouville equation, and in this way their general solutions are obtained.Comment: This is a contribution to the Proc. of the Seventh International
Conference ''Symmetry in Nonlinear Mathematical Physics'' (June 24-30, 2007,
Kyiv, Ukraine), published in SIGMA (Symmetry, Integrability and Geometry:
Methods and Applications) at http://www.emis.de/journals/SIGMA