Why isn't the answer working?

2 1993 p ( m o d 11 ) 2^{1993} \equiv p \pmod {11}

Find the second smallest non-negative integer p p such that the above congruence is satisfied.


The answer is 19.

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2 solutions

Sharky Kesa
Aug 15, 2014

Fermat's Little Theorem states that if p p is prime and gcd a , p = 1 \gcd{a, p} = 1 , then

a p 1 1 ( m o d p ) a^{p-1} \equiv 1 \pmod{p}

Using this theorem, we get this:

2 10 1 ( m o d 11 ) 2^{10} \equiv 1 \pmod{11}

Which further leads to

2 1993 2 3 8 ( m o d 11 ) 2^{1993} \equiv 2^3 \equiv 8 \pmod{11}

Hence, 8 8 is the smallest answer. But the question asks for the second smallest so we add 11 11 to the answer to get the actual answer as 19 19 .

Nice! Great problem! :D

Finn Hulse - 6 years, 10 months ago

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My last few problems have been to do with reading the problem. Many people would try 8 as their answer for this question first.

Sharky Kesa - 6 years, 10 months ago

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I noticed that! :D

Finn Hulse - 6 years, 10 months ago

Got the right answer in 3rd attempt after reading the name of the problem-" Why isn't the answer working?" And that's exactly what I thought.

Karthik Sharma - 6 years, 10 months ago

This was way overrated... Nice Problem Though!!

Mehul Arora - 6 years, 3 months ago
Akash Deep
Aug 17, 2014

i did it with simple congruence 2 5 1 ( m o d 11 ) 2 1990 1 ( m o d 11 ) 2 3 3 ( m o d 11 ) 2 1993 3 ( m o d 11 ) 2 1993 8 ( m o d 11 ) 8 19 ( m o d 11 ) 2 1993 19 ( m o d 11 ) { 2 }^{ 5\quad }\equiv \quad -1(mod\quad 11)\\ 2^{ 1990 }\quad \equiv \quad 1(mod\quad 11)\\ { 2 }^{ 3 }\quad \equiv \quad -3(mod\quad 11)\\ { 2 }^{ 1993 }\quad \equiv \quad -3(mod\quad 11)\\ { 2 }^{ 1993 }\quad \equiv \quad 8\quad (mod\quad 11)\\ 8\quad \equiv \quad 19\quad (mod\quad 11)\\ { 2 }^{ 1993 }\quad \equiv \quad 19\quad (mod\quad 11)\\ \quad

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