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GATE 2026 ST – Question 47

Multivariate Analysis · Multivariate Normal and Wishart Distributions · 2 marks · Multiple choice

Independent iid X,Y follow N_p(μ,Σ),Σ positive definite. Which follows a chi-square law as stated?

  1. (X−Y)ᵀΣ⁻¹(X−Y)/2 follows χ²_p
  2. 2(X−Y)ᵀΣ(X−Y) follows χ²_p
  3. (X−Y)ᵀΣ⁻¹(X−Y)/2 follows χ²_(2p)
  4. 2(X−Y)ᵀΣ(X−Y) follows χ²_(2p)

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Correct answer: (A) (X−Y)ᵀΣ⁻¹(X−Y)/2 follows χ²_p

Explanation

X−Y is N_p(0,2Σ). Standardize by (2Σ)^(−1/2) to obtain p independent standard normals. Their squared norm is (1/2)(X−Y)ᵀΣ⁻¹(X−Y), with **p degrees of freedom, A**.

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