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GATE 2024 MA – Question 51

Linear Algebra · Eigenvalues and Canonical Forms · 2 marks · Multiple select

T,S:ℝ⁴→ℝ⁴ are nonzero and nonidentity; T²=T. Which statements hold?

  1. T is necessarily invertible.
  2. If S²=S and ranks agree, T and S are similar.
  3. If S has only eigenvalues 0 and 1, T and S are similar.
  4. T is necessarily diagonalizable.

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Correct answer: (B) If S²=S and ranks agree, T and S are similar.; (D) T is necessarily diagonalizable.

Explanation

Minimal polynomial of T divides x(x−1), so T is diagonalizable with rank(T) copies of 1 and the remaining copies 0. This proves D and the rank criterion B for another projection. A fails since a nonidentity projection has a kernel. Eigenvalue sets alone do not fix multiplicities or Jordan blocks, refuting C. **B,D**.

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