@article{AFRW21,
  journal = {Pacific Journal of Mathematics},
  number = {2},
  volume = {313},
  year = {2021},
  title = {Ackermann and Goodstein go functorial},
  pages = {251-291},
  author = {Juan P. Aguilera and Anton Freund and Michael Rathjen and Andreas
            Weiermann},
}