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:
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?