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GATE 2025 MA – Question 11

Linear Algebra · Eigenvalues and Canonical Forms · 1 mark · Multiple choice

S is the set of unit vectors w∈ℝ³ for which $A=(1,-3,2)^Tw^T$ is diagonalizable. Which statement holds?

  1. S is compact and connected.
  2. S is neither compact nor connected.
  3. S is compact but disconnected.
  4. S is connected but not compact.

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Correct answer: (B) S is neither compact nor connected.

Explanation

Let v=(1,−3,2)ᵀ. Then $A^2=(w^Tv)A$. If wᵀv ≠ 0, the minimal polynomial divides t(t−wᵀv), with distinct roots, so A is diagonalizable. If wᵀv = 0, A is nonzero nilpotent and not diagonalizable. Thus S is the unit sphere minus the great circle wᵀv=0: two disjoint open hemispheres. It is disconnected and not closed/bounded-closed compact. **B**.

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