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Let \(M \sim W_{p}(n, \Sigma)\) and \(CC' = \Sigma\) where C is a non-singular matrix. Then \(C^{-1}MC^{-1}\) is distributed as :

  • A.\(W_{p}(n, C)\)
  • B.\(W_{p}(n, \Sigma^{-1})\)
  • C.\(W_{p}(n, I_{p})\)
  • D.\(W_{p}(n, C^{-1})\)

Answer: C

The distribution is \(W_{p}(n, I_{p})\).

If \(M \sim W_{p}(n, \Sigma)\), then \(BMB' \sim W_{p}(n, B\Sigma B')\) for any non-singular \(B\). Taking \(B = C^{-1}\):

\[ C^{-1}\Sigma(C^{-1})' = C^{-1}CC'(C')^{-1} = I_{p} \]

The paper prints \(C^{-1}MC^{-1}\); the result as keyed holds for \(C^{-1}M(C^{-1})'\), which is the same thing when C is symmetric, for instance \(C = \Sigma^{1/2}\). The scale matrix becomes the identity, not C, \(C^{-1}\) or \(\Sigma^{-1}\).

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