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GATE 2020 ST – Question 39

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

A4×4 matrix has minimal polynomial(x−1)²(x−3)². Which statement is FALSE?

  1. Its eigenvalues are1 and3.
  2. Eigenvalue1 has algebraic multiplicity3.
  3. It is not diagonalizable.
  4. Its trace is8.

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Correct answer: (B) Eigenvalue1 has algebraic multiplicity3.

Explanation

The minimal polynomial already has degree4, equal to the matrix size, and divides the characteristic polynomial, so they coincide. Both eigenvalues therefore have algebraic multiplicity2, making B false. Trace=2(1)+2(3)=8. Repeated factors in the minimal polynomial prevent diagonalizability.

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