A

David

Darling

saddle

A saddle is a type of surface that is neither a peak nor a valley but still has a zero gradient. Saddle points are situated such that moving in one direction takes one uphill, while moving in another direction would be downhill. A saddle function is a function f (x, y) of two vectors x and y (which typically lie in different vector spaces) that is concave up in x and concave down in y.