Compute log(1+x) accurately for small values of x
y = log1p(x)
y = log1p(x) computes log(1+x),
compensating for the roundoff in 1+x. log1p(x) is
more accurate than log(1+x) for small values of
x. For small x, log1p(x) is
approximately x, whereas log(1+x) can
be zero.