Extra parameter for DirichletDistribution

*To*: mathgroup at smc.vnet.net*Subject*: [mg121912] Extra parameter for DirichletDistribution*From*: Todd <johnson.todd.r at gmail.com>*Date*: Thu, 6 Oct 2011 04:24:18 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com

Can anyone tell me why DirichletDistribution in Mathematica 8 requires k+1 parameters for a k-dimensional distribution and why the extra parameter cannot be 0, which would give results similar to other software packages that require only k parameters. The function of the extra parameter is clear from the mean of the 2D distribution given in the help file: Namely, the mean of DirichletDistribution[a1, a2, a3] is {a1/ (a1+a2+a3), {a2/(a1+a2+a3)}. In this case, setting a3 to 0 would give me the desired result, but the function requires a3 to be positive. I noticed that a definition of DirichletDistribution from an older Wolfram tutorial required only k parameters for a k-dimensional distribution. Todd