code: def Exponential(x, a, tau, y0):
      
      return np.exp(x / tau) * a + y0
docs: Exponential function
  
      Inputs:
      -------
          ``x``: independent variable
          ``a``: scaling factor
          ``tau``: time constant
          ``y0``: additive constant
  
      Formula:
      --------
          ``a*exp(x/tau)+y0``