1. Quantum States of Black Holes;Horowitz,1997
2. Solitons,1980
3. for references to the early work on sine-Gordon by Enneper, Bäcklund, etc., see the contribution to this volume by R.K. Bullough.
4. 2-manifolds of constant curvature, 3-parameter isometry groups and bäcklund transformations
5. Quantum Theory of Gravity;Jackiw,1984