It is well known that smooth saddle surfaces have negative Gauss curvature. In this survey talk
we will discuss couple of generalizations of this statement for non-smooth surfaces.
Links:
[1] http://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] http://www.mpim-bonn.mpg.de/node/3444
[3] http://www.mpim-bonn.mpg.de/node/3050