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A geometric stable distribution or geo-stable distribution is a type of probability distribution. The geometric stable distribution may be symmetric or asymmetric. A symmetric geometric stable distribution is also referred to as a Linnik distribution. The common Laplace distribution is a special case of the geometric stable distribution and of a Linnik distribution. The geometric stable distribution has applications in finance theory.

Characteristics
For most geometric stable distributions, the probability density function and cumulative distribution function have no closed form solution. But a geometric stable distribution can be defined by its characteristic function, which has the form:



\varphi(t;\alpha,\beta,\lambda,\mu) = [1+\lambda^{\alpha}|t|^{\alpha} \omega - i \mu t]^{-1} $$

where $$\omega = \begin{cases} 1 - i\tan\tfrac{\pi\alpha}{2} \beta \textrm{sign}(t) & \text{if }\alpha \ne 1 \\ 1 + i\tfrac{2}{\pi}\beta\log|t| \textrm{sign}(t) & \text{if }\alpha = 1 \end{cases}$$

$$\alpha$$, which must be greater than 0 and less than or equal to 2, is the shape parameter or index of stability, which determines how heavy the tails are. Lower $$\alpha$$ corresponds to heavier tails.

$$\beta$$, which must be greater than or equal to -1 and less than or equal to 1, is the skewness parameter. When $$\beta$$ is negative the distribution is skewed to the left and when $$\beta$$ is positive the distribution is skewed to the right. When $$\beta$$ is zero the distribution is symmetric, and the characteristic function reduces to:



\varphi(t;\alpha, 0, \lambda,\mu) = [1+\lambda^{\alpha}|t|^{\alpha} - i \mu t]^{-1} $$

The symmetric geometric stable distribution with $$\mu=0$$ is also referred to as a Linnik distribution. A completely skewed geometric stable distribution, that is with $$\beta=1$$, $$\alpha<1$$, with $$\mu$$ is also referred to as a Mittag–Leffler distribution. Although $$\beta$$ determines the skewness of the distribution, it should not be confused with the typical skewness coefficient or 3rd standardized moment, which in most circumstances is undefined for a geometric stable distribution.

$$\lambda>0$$ is the scale parameter and $$\mu$$ is the location parameter.

When $$\alpha$$=2, $$\beta$$=0 and $$\mu$$=0 (i.e., a symmetric geometric stable distribution or Linnik distribution with $$\alpha$$=2), the distribution becomes the symmetric Laplace distribution, which has a probability density function is


 * $$f(x|0,\lambda) = \frac{1}{2\lambda} \exp \left( -\frac{|x|}{\lambda} \right) \,\!$$

The Laplace distribution has a variance equal to $$2\lambda^2$$. However, for $$\alpha<2$$ the variance of the geometric stable distribution is infinite.

Relationship to the stable distribution
The stable distribution has the property that if $$X_1, X_2,...X_n$$ are independent, identically distributed random variables taken from a stable distribution, the sum $$Y = a_n (X_1 + X_2 + ... + X_n) + b_n$$ has the same distribution as the $$X_i$$s for some $$a_n$$ and $$b_n$$.

The geometric stable distribution has a similar property, but where the number of elements in the sum is a geometrically distributed random variable. If if $$X_1, X_2,...X_{Np}$$ are independent, identically distributed random variables taken from a geometric stable distribution, the limit of the sum $$Y = a_{Np} (X_1 + X_2 + ... + X_{Np}) + b_{Np}$$ approaches the distribution of the $$X_i$$s for some $$a_{Np}$$ and $$b_{Np}$$ as p approaches 0, where $$N_p$$ is a random variable independent of the $$X_i$$s taken from a geometric distribution with parameter p. In other words:


 * $$\Pr(N_p = n) = (1 - p)^{n-1}\,p\,$$

There is also a relationship between the stable distribution characteristic function and the geometric stable distribution characteristic function. The stable distribution has a characteristic function of the form:



\Phi(t;\alpha,\beta,\lambda,\mu) = \exp\left[~it\mu\!-\!|c t|^\alpha\,(1\!-\!i \beta\,\textrm{sign}(t)\Omega)~\right] $$

where $$\Omega = \begin{cases} \tan\tfrac{\pi\alpha}{2} & \text{if }\alpha \ne 1 \\ -\tfrac{2}{\pi}\log|t| & \text{if }\alpha = 1 \end{cases}$$

The geometric stable characteristic function can be expressed as:



\varphi(t;\alpha,\beta,\lambda,\mu) = [1 - \log(\Phi(t;\alpha,\beta,\lambda,\mu))]^{-1} $$