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GATE 2021 ST – Question 40

Matrix Theory · Eigenvalues and Matrix Decompositions · 2 marks · Multiple choice

For a real 3×3 A with I+A invertible,let B=(I+A)⁻¹(I−A). Which statement is true?

  1. If B is orthogonal,then A is invertible.
  2. If B is orthogonal,all eigenvalues of A are real.
  3. If B is skew-symmetric,then A is orthogonal.
  4. If B is skew-symmetric,det A=−1.

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Correct answer: (C) If B is skew-symmetric,then A is orthogonal.

Explanation

If B is skew-symmetric,I+B is invertible and A=(I−B)(I+B)⁻¹. Transposing and using Bᵀ=−B shows AᵀA=I,proving **C**. A=0 gives B=I and refutes A. Nonzero real skew-symmetric A has nonreal eigenvalues but its Cayley transform B is orthogonal,refuting B. For skew B,the Cayley transform A has determinant +1,not −1.

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