### _f2xm1 -- Calculate (2**x) - 1

Format

>     #include <builtin.h>
>     double _f2xm1(double x);

Language Level: Extension
_f2xm1 calculates 2 raised to the power of x, minus 1 ((2**x)-1). The variable x must be in the range -1 to 1. This instruction is designed to produce very accurate results when x is close to 0. Large errors may occur for operands with magnitudes close to 1. You can exponentiate values other than 2 by using the formula x**y = 2**(y * log2(x)).

Because it is a built-in function and has no backing code in the library:

- You cannot take the address of _f2xm1.
- You cannot parenthesize a _f2xm1 function call, because parentheses specify a call to the backing code for the function in the library.

Return Value
This function returns the value of the formula (2**x)-1.

Example
This example calculates (2**x)-1 after prompting the user for a value for x.

    #include <builtin.h>
    #include <stdio.h>

    int main(void)
    {
       double x;

       printf("Enter a value for x:\n");
       scanf("%lf", &x);
       printf("(2**(%lf)) - 1 is %lf.\n", x, _f2xm1(x));
       return 0;

       /**************************************************
          Assuming you enter : 0.001

          The output should be :

          (2**(0.001000)) - 1 is 0.000693.
       **************************************************/
    }

_fyl2x -- Calculate y * log2(x)
_fyl2xp1 -- Calculate y * log2(x + 1)
<builtin.h>