I need a concave function in 2 variables X and Y for some modelling I'm doing.
For 0<x<1 and 0<y<1, the function needs to satisfy the following:
f(x,y) < 0 for x<0.5 and/or y<0.5
f(x,y) = 0 for x=0.5 and y=0.5
f(x,y) > 0 for x>0.5 and y>0.5
MUST be concave.. i.e. second order partials...