The GATE Grind

GATE 2020 MA – Question 40

Topology · Connectedness and Compactness · 2 marks · Multiple choice

Equip the power set P(R) with $d(U,V)=\sup_{x\in\mathbb R}|\chi_U(x)-\chi_V(x)|$. The set $\{\{m\}:m\in\mathbb Z\}$ is

  1. Bounded but not totally bounded.
  2. Totally bounded but not compact.
  3. Compact.
  4. Unbounded.

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Show answer and explanation

Correct answer: (A) Bounded but not totally bounded.

Explanation

The distance between distinct subsets is 1, since their characteristic functions differ somewhere. Thus the metric is discrete and this family of singleton subsets has diameter 1, so it is bounded. A ball of radius less than 1 contains only one family member. Infinitely many such balls are needed to cover the family, so it is not totally bounded. Select A.

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