“…For this reason, the Gauss equation of these surfaces is related with the sine-Gordon equation, which is known to be integrable in the sense of the soliton theory. Recently there has been an increasing interest in studying the Gauss equation of these surfaces using the theory of integrable systems: [3,8,9,10,11,17,19]. For our purposes, we need to extend the notion of constant astigmatism surfaces in any 3-dimensional Riemannian space.…”