Jon was an exceptional person in many respects: a mathematician with an unbelievably large spectrum and of considerable charisma, generosity and humor. He always tried to avoid conflicts, seeking mediation where possible. I appreciated his open-mindedness and his undogmatic views on many issues (not only mathematical issues). I will forever be personally grateful for what he taught me and for his friendship during 38 years. [Michel Thera, Universite de Limoges, France]