The GATE Grind

GATE 2023 MA – Question 16

Real Analysis · Measure and Integration · 1 mark · Multiple choice

Let N⊆ℝ be non-Lebesgue-measurable. P:the irrational points of N form a measurable set. Q:the boundary of N has positive Lebesgue outer measure. Choose truth values.

  1. Both P and Q true
  2. P false, Q true
  3. P true, Q false
  4. Both false

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Show answer and explanation

Correct answer: (B) P false, Q true

Explanation

Removing a countable set cannot turn a nonmeasurable set into a measurable one: otherwise adding back N∩ℚ would make N measurable. If the boundary had outer measure zero, N would differ from its open interior only by a subset of a null set, hence be measurable by completeness. Thus P false and Q true. **B**.

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