Hi

I'm trying to learn this specific proof on the Equality of Mixed Partials and I pretty much understand the proof, but I need one clarification.

The proof is posted here:

http://homepage.smc.edu/nestler_andr...11clairaut.pdf
I am confused on what

means. At first I thought it was the change in h, but the parenthesis lead me to believe that it stands for a function of h. It would make sense to me if this was actually just a function of h, but I am not one-hundred percent sure.

Could someone tell me what

represents?

Thank you.