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The smallest positive prime (say p )) such that \( \Large 2^{p}-1 \) is not a prime is

A) 5
B) 11
C) 17
D) 29

Correct Answer:


B) 11

Description for Correct answer:
Taking p = 5

\( \Large 2^{p} \) -1= \( \Large 2^{5} \) - 1 = 31 which is prime

Taking p = 11

\( \Large 2^{p} \) -1=\( \Large 2^{11} \) -1=2047

Since, 2047 is divisible by 23, so it is not prime.

Thus, required least positive prime number is 11.

Part of solved Number System questions and answers : >> Aptitude >> Number System

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