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Part BCSIR NET June 2024bilinear-form-is-not-an-inner-product

Bilinear form is not an inner product

Let A = ((0, 1, 0, 0), (1, 0, 0, 0), (0, 0, 1, 1), (0, 0, 1, 1)), and consider the symmetric bilinear form on given by ⟨v, w⟩ = vᵗAw, for . Which of the following statements is true?

  1. A.A is invertible
  2. B.There exist non-zero vectors v, w such that ⟨v, w⟩ = 0
  3. C.⟨u, v⟩ ≠ ⟨u, w⟩ for all non-zero vectors u, v, w with v ≠ w
  4. D.Every eigenvalue of is positive

You have the answer. Trap Analysis is why the other three were written.

Not a worked solution repeated four times — the specific reasoning error each wrong option was built to reward.

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50 are analysed free — try those first.

Related counterexample: Every real symmetric matrix is positive definite if det > 0

More on this topic

The chapter behind this: Quadratic forms, signature and definiteness — free to read

From Inner Product Spaces and FormsQuadratic forms, positive definiteness, Sylvester's law

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