In probability theory, a hyperexponential distribution is a continuous probability distribution whose probability density function of the random variable X is given by f X = ∑ i = 1 n f Y i p i, {\displaystyle f_{X}=\sum _{i=1}^{n}f_{Y_{i}}\;p_{i},} where each Yi is an exponentially distributed random variable with rate parameter λi, and pi is the probability that X will take on the form of the exponential distribution with rate λi. Wikipedia