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GATE 2025 MA – Question 17

Functional Analysis · Bounded Linear Operators and Duality · 1 mark · Multiple choice

X={f∈C[0,1]:f(0)=f(1)=0} with sup norm; Y=C[0,1] with L² norm. On their closed unit balls U_X,U_Y, define Tf=Sf=∫₀¹f(t)dt. S1: sup over U_X of |Tf| is attained. S2: sup over U_Y of |Sf| is attained. Choose the truth values.

  1. S1 true, S2 false
  2. S2 true, S1 false
  3. Both true
  4. Neither true

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Correct answer: (B) S2 true, S1 false

Explanation

In X the supremum is 1: continuous plateau functions with narrow endpoint ramps approach it, but any function bounded by 1 and zero at both endpoints has strictly smaller absolute integral. S1 is false. In Y, Cauchy–Schwarz gives |∫f|≤‖f‖₂≤1, attained by f≡1. S2 is true. **B**.

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