https://journals.aps.org/prd/pdf/10.1103/PhysRevD.98.030001
https://en.wikipedia.org/wiki/Koide_formula
In[]:=
electron
PARTICLE

mass
+
tau
PARTICLE

mass

+
muon
PARTICLE

mass

Sqrt
electron
PARTICLE

mass
+Sqrt
tau
PARTICLE

mass

+Sqrt
muon
PARTICLE

mass

^2
Out[]=
0.66666
http://inspirehep.net/record/1327002/files/Thesis-2008-Goffinet.pdf
https://arxiv.org/pdf/hep-ph/0505220.pdf