(where inverse cosh is taken to be positive), and d E / d M goes to infinity at these values of M . This means that the radius of convergence of the Maclaurin series is cosh − − 1 ⁡ ⁡ ( 1 / e ) − − 1 − − e 2 and the series will not converge for values of M larger than this. The series can also be used for the hyperbolic case, in which case the radius of convergence is cos − − 1 ⁡ ⁡ ( 1 / e ) − − e 2 − − 1 . The series for when e = 1 converges when M < 2 π π .