I was looking at solutions for a problem but I noticed it said $\langle2\rangle$ and was unsure of what that meant.
From Homework 5 (PDF)
From the context of the exercise, it appears that $\langle x\rangle$ is meant to denote a canonical representation of the integer $x$ as a bitstring of length $n$.
If this were not the case, those inputs and outputs would not be in the correct domain and range respectively.
Likely this notation was defined somewhere in the lecture notes, but I cannot find them at the moment.