expm1
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header <math.h>
  | ||
| float       expm1f( float arg ); | (since C99) | |
| double      expm1( double arg ); | (since C99) | |
| long double expm1l( long double arg ); | (since C99) | |
Computes the e (Euler's number, 2.7182818) raised to the given power arg, minus 1. This function is more accurate than the expression exp(arg)-1 if arg is close to zero.
[edit] Parameters
| arg | - | floating point value | 
[edit] Return value
earg
-1
If the result is too large for the underlying type, range error occurs and HUGE_VAL is returned.
[edit] See also
| returns e raised to the given power (ex) (function) | |
| (C99) | returns 2 raised to the given power (2x) (function) |