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GATE 2025 ST – Question 12

Calculus · Sequences and Series · 1 mark · Multiple choice

Which statement about countability is correct?

  1. The union over n≥0 of {Σᵢ₌₀ⁿ10ⁱa_i:a_i∈{1,2}} is uncountable.
  2. The set {Σₙ₌₁∞a_n/10ⁿ:a_n∈{1,2}}⊂(0,1) is uncountable.
  3. An uncountable family of pairwise disjoint open intervals in ℝ exists.
  4. All intervals with rational endpoints form an uncountable set.

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Correct answer: (B) The set {Σₙ₌₁∞a_n/10ⁿ:a_n∈{1,2}}⊂(0,1) is uncountable.

Explanation

A is a countable union of finite sets. B is in bijection with all infinite binary sequences, an uncountable set; its decimal representations using only 1,2 are unique. Each disjoint nonempty open interval contains a different rational, so C is countable. Rational endpoint pairs form a subset of ℚ², refuting D. **B**.

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