BibTeX: 
@article{Ackermanns_not_PRAFP,
author = {Lawrence C. Paulson},
title = {Ackermann's Function Is Not Primitive Recursive},
journal = {Archive of Formal Proofs},
month = mar,
year = 2022,
note = {\url{https://isaafp.org/entries/Ackermanns_not_PR.html},
Formal proof development},
ISSN = {2150914x},
}
