code: def _psi(self,m):
          
          if self.twominusalpha == 0.:
              return -2.*self.a**2*(self.a/m)**self.betaminusalpha\
                        /self.betaminusalpha\
                  *special.hyp2f1(self.betaminusalpha,
                                  self.betaminusalpha,
                                  self.betaminusalpha+1,
                                  -self.a/m)
          else:
              return -2.*self.a**2\
                  *(self.psi_inf-(m/self.a)**self.twominusalpha\
                        /self.twominusalpha\
                        *special.hyp2f1(self.twominusalpha,
                                        self.betaminusalpha,
                                        self.threeminusalpha,
                                        -m/self.a))
docs: \psi(m) = -\int_m^\infty d m^2 \rho(m^2)