MAT8886 the Dirichlet distribution
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In the course, still introducing some concept of dependent distributions, we will talk about the Dirichlet distribution (which is a distribution over the simplex of ). Let
denote the Gamma distribution with density (on
)
Let denote independent
random variables, with
. Then
where
has a Dirichlet distribution with parameter . Note that
has a distribution in the simplex of
,
and has density
We will write .
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or
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and finally, below,
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Note that marginal distributions are also Dirichlet, in the sense that if
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> library(MCMCpack) > alpha=c(2,2,5) > x=seq(0,1,by=.05) > vx=rep(x,length(x)) > vy=rep(x,each=length(x)) > vz=1-x-vy > V=cbind(vx,vy,vz) > D=ddirichlet(V, alpha) > persp(x,x,matrix(D,length(x),length(x))(to plot the density, as figures above). Note that we will come back on that distribution later on so-called Liouville copulas (see also Gupta & Richards (1986)).
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