Mittag-Leffler Distribution

The Mittag-Leffler distributions are two families of probability distributions on the half-line ,\infty). They are parametrized by a real \alpha \in (0, 1/math> or $\alpha \in$, 1/math>. Both are defined with the Mittag-Leffler function, named after Gösta Mittag-Leffler.

The Mittag-Leffler function

For any complex $\alpha$ whose real part is positive, the series

:$E_\alpha (z) := \sum_^\infty \frac$

defines an entire function. For $\alpha = 0$, the series converges only on a disc of radius one, but it can be analytically extended to $\mathbb \setminus \$.

First family of Mittag-Leffler distributions

The first family of Mittag-Leffler distributions is defined by a relation between the Mittag-Leffler function and their cumulative distribution functions.

For all $\alpha \in (0, 1]$, the function $E_\alpha$ is increasing on the real line, converges to $0$ in $- \infty$, and $E_\alpha (0) = 1$. Hence, the function $x \mapsto 1-E_\alpha (-x^\alpha)$ is the cumulative distribution function of a probability measure on the non-negative real numbers. The distribution thus defined, and any of its multiples, is called a Mittag-Leffler distribution of order $\alpha$.

All these probability distributions are Absolute_continuity, absolutely continuous. Since $E_1$ is the exponential function, the Mittag-Leffler distribution of order $1$ is an exponential distribution. However, for $\alpha \in (0, 1)$, the Mittag-Leffler distributions are heavy-tailed. Their Laplace transform is given by:

:$\mathbb (e^) = \frac,$

which implies that, for $\alpha \in (0, 1)$, the expectation is infinite. In addition, these distributions are geometric stable distributions. Parameter estimation procedures can be found here.

Second family of Mittag-Leffler distributions

The second family of Mittag-Leffler distributions is defined by a relation between the Mittag-Leffler function and their moment-generating functions.

For all $\alpha \in$, 1/math>, a random variable $X_\alpha$ is said to follow a Mittag-Leffler distribution of order $\alpha$ if, for some constant $C>0$,

:$\mathbb (e^) = E_\alpha (Cz),$

where the convergence stands for all $z$ in the complex plane if $\alpha \in (0, 1]$, and all $z$ in a disc of radius $1/C$ if $\alpha = 0$.

A Mittag-Leffler distribution of order $0$ is an exponential distribution. A Mittag-Leffler distribution of order $1/2$ is the distribution of the absolute value of a gaussian, normal distribution random variable. A Mittag-Leffler distribution of order $1$ is a degenerate distribution. In opposition to the first family of Mittag-Leffler distribution, these distributions are not heavy-tailed.

These distributions are commonly found in relation with the local time of Markov processes.

References

{{DEFAULTSORT:Mittag-Leffler distribution
Continuous distributions
Geometric stable distributions
Probability distributions with non-finite variance