Koide formula
The Koide formula is an unexplained empirical equation proposed by Yoshio Koide ([ko.i.de], koe-ee-day) in 1981. It relates the masses of the charged leptons, the electron, muon and tau in the following way: Q = m e + m μ + m τ ( m e + m μ + m τ ) 2 = 2 3 , {\displaystyle Q={\frac {m_{\text{e}}+m_{\mu }+m_{\tau }}{\left({\sqrt {m_{\text{e}}}}+{\sqrt {m_{\mu }}}+{\sqrt {m_{\tau }}}\right)^{2}}}={\frac {2}{3}},} This exact value of 2 3 {\displaystyle {\frac {2}{3}}} is still within experimental bounds as of 2014.