# Need a concave function

1. Jun 22, 2011

### johnd17

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 wrt X and Y must both be < 0.

My maths was never that good and I can't seem to make any progress here.. can anyone help out? Thanks

2. Jun 22, 2011

### I like Serena

Hi johnd17! Welcome to PF!

$$f(x,y) = \ln(x + \frac 1 2) + \ln(y + \frac 1 2)$$