After all the students have left the classroom, a professor notices that four copies of the text were left under desks. At the beginning of the next lecture, the professor distributes the four books in a completely random fashion to each of the four students (we'll call them 1, 2, 3, and 4) who claim to have left books. One possible outcome is that 1 receives 2's book, 2 receives 4's book, 3 receives their own book, and 4 receives 1's book. This outcome can be abbreviated as (2, 4, 3, 1). Let X denote the number of students who receive their own book. Determine the pmf of X, and find the probability that at least 2 students receive their own books.