= Hypoexponential distribution
{wiki}
A hypoexponential distribution is the distribution of a sum of <independent> variables with <exponential distributions>. For two positive unequal rates $\lambda,\mu$, <convolution of independent random variables> gives the <probability density function>
$$
f(s)=\frac{\lambda\mu}{\mu-\lambda}(e^{-\lambda s}-e^{-\mu s}),\qquad s\geq0.
$$
It is zero for negative $s$. Its equal-rate limit is the shape-two <gamma distribution> with rate $\lambda$.
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