The GATE Grind

GATE 2026 MA – Question 42

Functional Analysis · Bounded Linear Operators and Duality · 2 marks · Multiple choice

On ℓ∞ with supremum norm, $T(x_1,x_2,\ldots)=(x_1,x_2/2,x_3/3,\ldots)$. Which statement holds?

  1. T is bounded but not injective.
  2. T is injective but not bounded.
  3. T is bounded and its inverse on its range exists but is unbounded.
  4. T and its inverse on its range are bounded.

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (C) T is bounded and its inverse on its range exists but is unbounded.

Explanation

‖Tx‖∞ ≤ ‖x‖∞, and nonzero scaling in every coordinate makes T injective. On the range, inverse multiplies coordinate n by n. For the unit coordinate vector e_n, ‖Te_n‖∞ = 1/n while ‖e_n‖∞ = 1, so inverse norms have ratios n without bound. **C**.

More Bounded Linear Operators and Duality questions

All Bounded Linear Operators and Duality questions →