One of the simplest is the Ackermann function. Citation author given name Jean-Francois title of source Understanding Formal Methods url of source https://books.google.com/books?id=rUudIPZD-B0C&pg=PA61 year 2003 quote There are total functions that cannot be defined by a primitive recursive presentation, but they are not that easy to find. One of the simplest is the Ackermann function. last1 Monin last2 Hinchey author2 Hinchey first1 Jean-Francois first2 M. G. page 61 publisher Springer isbn 9781852332471 ISBN 9781852332471 1 2 Wilhelm Ackermann (1928).