For most people it's algebra, especially around the point when all the numbers start disappearing to be replaced by letters. For some it's the more abstract stuff, like the definition of a function, or limits. But for Light O, it's NON-LINEAR FUNCTIONS.

NNescio said:

For most people it's algebra, especially around the point when all the numbers start disappearing to be replaced by letters. For some it's the more abstract stuff, like the definition of a function, or limits. But for Light O, it's NON-LINEAR FUNCTIONS.

or for us who took Trigonometry back in 1st year high school... cursed curved o'tips