The Weibull distribution with shape parameter \(a\) and
scale parameter \(\sigma\) has density given by
$$
f(x) = (a/\sigma) {(x/\sigma)}^{a-1} \exp (-{(x/\sigma)}^{a})
$$
for \(x \ge 0\).
The generation algorithm uses fast numerical inversion. The parameters
lb and ub can be used to generate variates from
the Weibull distribution truncated to the interval (lb,ub).