code: def BorueErukhimovich(q, C, r0, s, t):
      
      x = q * r0
      return C * (x ** 2 + s) / ((x ** 2 + s) * (x ** 2 + t) + 1)
docs: Borue-Erukhimovich model of microphase separation in polyelectrolytes
  
      Inputs:
      -------
          ``q``: independent variable
          ``C``: scaling factor
          ``r0``: typical el.stat. screening length
          ``s``: dimensionless charge concentration
          ``t``: dimensionless temperature
  
      Formula:
      --------
          ``C*(x^2+s)/((x^2+s)(x^2+t)+1)`` where ``x=q*r0``
  
      Literature:
      -----------
          o Borue and Erukhimovich. Macromolecules (1988) 21 (11) 3240-3249
          o Shibayama and Tanaka. J. Chem. Phys (1995) 102 (23) 9392
          o Moussaid et. al. J. Phys II (France) (1993) 3 (4) 573-594
          o Ermi and Amis. Macromolecules (1997) 30 (22) 6937-6942