Decay constant of pion in the quark model
  To describe pion as a bound state of two quark, when their current masses are non equal to zero, is considered. In the framefowrk of the potential approximate the Schwinger-Dyson and Bethe-Salpeter equations are solved. The quark-antiquark interaction as sum of the oscillator and Coulomb potential is chosen. The mass and constant of pion are calculated. 
Obid Juraev, Tulkun Nasyrov