code: def _tab_P(P):
      
      I = [0, 1, 2, -1, -2]
      n = [0.154793642129415e4, -0.187661219490113e3, 0.213144632222113e2,
           -0.191887498864292e4, 0.918419702359447e3]
  
      Pr = P/1
      T = 0
      for i, ni in zip(I, n):
          T += ni * log(Pr)**i
      return T
docs: Define the boundary between Region 3a-3b, T=f(P)
  
      Parameters
      ----------
      P : float
          Pressure, [MPa]
  
      Returns
      -------
      T : float
          Temperature, [K]
  
      References
      ----------
      IAPWS, Revised Supplementary Release on Backward Equations for Specific
      Volume as a Function of Pressure and Temperature v(p,T) for Region 3 of the
      IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water
      and Steam, http://www.iapws.org/relguide/Supp-VPT3-2016.pdf, Eq. 2
  
      Examples
      --------
      >>> _tab_P(40)
      693.0341408