code: def ExcludedVolumeChain(q, Rg, nu):
      
      u = (q * Rg) ** 2 * (2 * nu + 1) * (2 * nu + 2) / 6.
      return (u ** (0.5 / nu) * gamma(0.5 / nu) * gammainc(0.5 / nu, u) -
              gamma(1. / nu) * gammainc(1. / nu, u)) / (nu * u ** (1. / nu))
docs: Scattering intensity of a generalized excluded-volume Gaussian chain
  
      Inputs:
      -------
          ``q``: independent variable
          ``Rg``: radius of gyration
          ``nu``: excluded volume exponent
  
      Formula:
      --------
          ``(u^(1/nu)*gamma(0.5/nu)*gammainc_lower(0.5/nu,u)-
              gamma(1/nu)*gammainc_lower(1/nu,u)) / (nu*u^(1/nu))``
          where ``u = q^2*Rg^2*(2*nu+1)*(2*nu+2)/6`` is the reduced scattering
          variable, ``gamma(x)`` is the gamma function and ``gammainc_lower(x,t)``
          is the lower incomplete gamma function.
  
      Literature:
      -----------
          SASFit manual 6. nov. 2010. Equation (3.60b)