Dear all,

I am looking a function of two variables, f(x,y), with the following features:

- The first derivatives are positive, f
_{x}>0, f_{y}>0- The second derivatives are negative, f
_{x}_{x}<0, f_{y}_{y}<0- The mixed second derivatives is negative, f
_{xy}<0

I have trouble coming up a function having these features, especially the third one.

Any help will be highly appreciated.

Thanks.

Jack