The GATE Grind

GATE 2021 ST – Question 43

Probability · Events and Conditional Probability · 2 marks · Multiple choice

On sample space ℕ,let P({n})=a_n. Which statement is NOT true?

  1. a_n→0.
  2. Σ√a_n converges.
  3. For each positive integer k,there are k disjoint events whose union has probability below .001.
  4. There are strictly increasing events A_i whose infinite union has probability below .001.

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Show answer and explanation

Correct answer: (B) Σ√a_n converges.

Explanation

The probabilities form a convergent nonnegative series with sum one,so a_n→0. B need not hold:take a_n=6/(π²n²),whose square roots form a divergent harmonic multiple. For C and D,choose a far enough tail of total probability below .001,then choose singleton events or increasing finite initial portions of that tail. **B**.

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