Using (2259.3 Β± 11.1) Γ 106π(2π) events acquired with the BESIII detector, the branching
fraction of π(2π) β π+πβ is measured with improved precision to be π΅π(π(2π) β π+πβ) =
(3.240Β±0.023Β±0.081)Γ10β3,wherethefirstandseconduncertaintiesarestatisticalandsystematic,
respectively, which is consistent with the world average value within one standard deviation. This
value, along with those for the branching fractions of the π(2π) decaying into π+πβ and π+πβ, is
in good agreement with the relation predicted by the sequential lepton hypothesis.

