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GATE 2021 ST – Question 28

Convergence of Random Variables · Modes of Convergence · 1 mark · Numerical answer

For each α>0,independent Bernoulli variables X_n^(α) have P(X_n^(α)=1)=1/n^(2α). Let S={α>0:X_n^(α)→0 almost surely}. Find inf S,to two decimals.

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Correct answer: 0.50 to 0.50

Explanation

Almost-sure convergence to zero for a zero-one sequence means only finitely many ones. By independence and Borel–Cantelli,this occurs exactly when Σn^(−2α) converges,i.e. α>1/2. Thus S=(1/2,∞),and its infimum is **0.50**,though the endpoint itself is excluded.

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