say "hey I saw you in my class and I wanted to ask you if you can use partial differential equations on a multivariate Markov exponential while accounting for the nonlinear skew of the stochastic displacement"

...It really doesn't matter what you say as long as your body language is good and you are confident. From your question you are obviously new, you are going to have challenges all throughout the pick up process, for now you just need to approach girls all the time as your first step and get used to that.