As I’m currently studying for a BSc in Mathematics with the Open University and I am trying to teach myself Common Lisp, I decided to try and create a function that outputs the Fibonacci nth element. Here it is, I think is a nice example to illustrate how recursion works in Lisp.
(defun fibonacci (n)
    (cond
        ((> n 1) (+ (fibonacci (- n 1)) (fibonacci (- n 2))))
        ((= n 1) 1)
        ((= n 0) 0)
        (T (print "invalid number"))))