# Proving An Inequality

1. Feb 13, 2016

### Physicist97

Hello!
Say we have an inequality that says that $f(x, y)>c$ where $f(x, y)$ is a function of two variables and $c$ is a constant. Assume that we know this inequality to be true when $x=a$ and $y=b$. If you show that the partial derivatives of $f(x, y)$ with respect to $x$ and $y$ are both greater than zero, does that prove that $f(x, y)>c$ whenever $x$ is greater than or equal to $a$ and $y$ is greater than or equal to $b$?

2. Feb 13, 2016

### Staff: Mentor

3. Feb 13, 2016

### Physicist97

The partial derivatives are positive in the regions $x>a$ and $y>b$. They could be positive everywhere, but the above is what I think is important to proving that inequality. I could be wrong, though.

4. Feb 13, 2016

### Staff: Mentor

The mexican hat potential is a counterexample. Only that the derivatives in (0,0) are zero. But then one could define a pole there.
All derivatives are positive, the function values let's say in a circle of radius r are all above c but not outside of it, i.e. for x,y > r.