Stable Distributions



1 Stable distributions

The distribution F (x; α, β) of a standard α-stable random variable in represen-
tation
S0 can be expressed as:

when α 6= 1 and x > ζ :

F (x; α, β) = c1 (α, β) +


sign(1 α ) 12 exp © - ( x - ζ ) α-1 V ( θ ; α,β )} dθ,
π      
-ξ

where


c 1( α,β )= 1П(П $’


α< 1,

α> 1,


when α 6= 1 and x = ζ :

F(χ; α,β) = 1 (П

π2

when α 6= 1 and x < ζ :

F(x; α, β) = 1 - F (-x; α, -β),

when α = 1:

∏ Λ r∙ —            —         π X ,            . ʌl

L2 exp {-e-V(θ;1 )} dθ, β> 0,

F( x ;1 )=S 1 + 1 arctan x,                   β = 0,

2π

. 1 - F(χ, 1, -β),

β<0.


Formula (1.5) requires numerical integration of the function g() exp{-g()},
where
g(θ; x,α,β) = (x - ζ)α-1 V(θ; α, β). The integrand is 0 at , increases
monotonically to a maximum of
1 at point θ* for which g(θ* ; x,α,β) = 1,
and then decreases monotonically to 0 at
(Nolan, 1997). However, in some
cases the integrand becomes very peaked and numerical algorithms can miss
the spike and underestimate the integral. To avoid this problem we need to
find the argument
θ* of the peak numerically and compute the integral as a
sum of two integrals: one from
to θ* and the other from θ* to .



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