This is not an often asked question but a very interesting one,
  because LISP itself is defined recursively and it's often the
  easiest way to articulate hard problems.

  There some fine documents about recursion at
  http://xarch.tu-graz.ac.at/autocad/lisp/ especially the
  http://xarch.tu-graz.ac.at/autocad/lisp/recursive.html tutorial by
  Dennis Shinn.

  It explains in great detail:

  (defun fact (n)
    (cond
      ((zerop n) 1)
      (T (* n (fact (1- n))))))

  Note: There's also a self-modifying example of this function
  explained at http://xarch.tu-graz.ac.at/autocad/lisp/self-mod.lsp

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