The Koide formula is (m_e + m_mu + m_tau)/(sqrt(m_e) + sqrt(m_mu) + sqrt(m_tau))^2 = 2/3, where m_e, m_mu and m_tau are the masses of the electron, the muon and of the tau lepton respectively. (This is halfway between the lowest and the highest possible value of (a+b+c)/(sqrt(a)+sqrt(b)+sqrt(c))^2, namely 1/3 and 1 respectively.) Using current values of the masses as of the latest Review of Particle Physics by the Particle Data Group (https://pdg.lbl.gov/2023/tables/rpp2023-sum-leptons.pdf), the left-hand side is 0.666 660 5 ± 0.000 006 7 (so, 0.9 standard deviations away from the Koide formula prediction).

Immediately resolves as NO if any future edition of the Review of Particle Physics before the market closing date lists a tau lepton mass more than 5 standard deviations away from the Koide formula prediction (namely 1776.969 MeV using current m_e and m_mu values, and unlikely to change enough to matter). Resolves as YES if the 2050 edition still lists a value within 2 standard deviations of the prediction. The closing date will be postponed if the 2050 RPP lists a value between 2 and 5 standard deviations of the prediction.

P.S.: I may resolve this YES early if some theory which predicts the Koide formula to be correct becomes such consensus among particle physicists that the RPP starts listing its prediction (or a weighted average including it, which would be pretty much the same) as the main value for the mass of the tau lepton.

It has always baffled me that particle physicists don’t talk about this more often. It would be an outrageous coincidence.