Four colors suffice

The "Four colors suffice" postmark used by the University of Illinois at Urbana-Champaign (UIUC) after the celebrated proof of the Four Colors Problem by Kenneth Appel and Wolfgang Haken [1]. 

According to [2], the proof "was greeted with enthusiasm [...] but also with skepticism, great disappointment and outright rejection. Their extensive use of computers was widely criticized, and raised philosophical questions as to whether a proof is valid if it cannot be checked by hands."

References.

[1] K. Appel and W. Haken: Every planar map is four colorable I-II, Illinois J. Math. 21, No. 3 (1977).
[2] Wolfgang Haken (1928-2022), Memorial Tribute, Notices of the AMS 70, Number 9 (2023). DOI: https://doi.org/10.1090/noti2781