BibTeX: 
@article{Three_CirclesAFP,
author = {Fox Thomson and Wenda Li},
title = {The Theorem of Three Circles},
journal = {Archive of Formal Proofs},
month = aug,
year = 2021,
note = {\url{https://isaafp.org/entries/Three_Circles.html},
Formal proof development},
ISSN = {2150914x},
}
