1. The problem statement, all variables and given/known data According to Wikipedia and one of my calculus textbooks: (I am used to this notation) But I've seen another source writing fxy=∂2f/∂x∂y=∂xyf which is totally inconsistent with the above. How come? Which one is standard? Is the second one wrong, or is it acceptable? The second one is driving me crazy...is it ever used anywhere? 2. Relevant equations N/A 3. The attempt at a solution N/A Thank you!