The expm1() function returns the exponential of the argument minus one, as a number, computed in a way that is accurate even when the value of the argument is close to zero. The corresponding mathematical notation is:
The constant e is Euler's number, occasionally termed Napier's constant, the base of the natural logarithms.
When the argument x has a small absolute value, the value is close to one, and computing the difference may produce a loss of accuracy. The function expm1() computes directly, without a loss of accuracy.
The true value of is 1.00000000005e-10 to about 32 significant digits. This example shows the superiority of expm1() in this case: