powi

Real raised to integer power.

SYNOPSIS:

double x, y, powi();
int n;

y = powi(x, n);

DESCRIPTION:

Returns an argument x raised to the nth power. The routine efficiently decomposes n as a sum of powers of two. The desired power is a product of two-to-the-kth powers of x. Thus to compute the 32767 power of x requires 28 multiplications instead of 32767 multiplications.

ACCURACY:
                      Relative error:
arithmetic   x domain   n domain # trials      peak         rms
    DEC       .04,26     -26,26    100000       2.7e-16     4.3e-17
    IEEE      .04,26     -26,26     50000       2.0e-15     3.8e-16
    IEEE        1,2    -1022,1023   50000       8.6e-14     1.6e-14

Returns MAXNUM on overflow, zero on underflow.