Often in my studies (economics) the assumption of a "well-behaved" function will be invoked. I don't exactly know what that entails (I think twice continuously differentiability is one of the requirements), nor do I know why this is necessary (though I imagine the why will depend on each case).
Can someone explain it to me, and if there is an explanation of the why as well, I would be grateful. Thanks!
EDIT: To give one example where the term appears, see this Wikipedia entry for utility functions, which says at one point:
In order to simplify calculations, various assumptions have been made of utility functions.
CES (constant elasticity of substitution, or isoelastic) utility
Most utility functions used in modeling or theory are well-behaved. They are usually monotonic, quasi-concave, continuous and globally non-satiated.
I might be wrong, but I don't think "well-behaved" means monotonic, quasi-concave, continuous and globally non-satiated. What about twice differentiable?