Makar-Limanov, Leonid Max Planck Institute for Mathematics, Max Planck Society;
https://doi.org/10.1307/mmj/1123090767 (Publisher version)
https://www.mpim-bonn.mpg.de/newpreprints?ppyear=2003&ppnum=80 (Preprint)
Bandman, T., & Makar-Limanov, L. (2005). Nonstability of the AK invariant. Michigan Mathematical Journal, 53(2), 263-281. doi:10.1307/mmj/1123090767.