A Pair of Primes

Let p , q p, q be primes greater than 7 such that q q divides 2 2 p + 2 p + 1 2^{2p} + 2^p + 1 . Then, q q can be expressed as

one less than a multiple of 6 p 6p None of these choices one more than a multiple of 5 p 5p one more than a multiple of 3 p 3p one less than a multiple of 4 p 4p one less than a multiple of 9 p 9p one more than a multiple of 9 p 9p one more than a multiple of 6 p 6p

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1 solution

Mathh Mathh
Aug 11, 2015

q ( 2 p 1 ) ( 2 2 p + 2 p + 1 ) = 2 3 p 1 = 8 p 1 q\mid \left(2^p-1\right)\left(2^{2p}+2^{p}+1\right)=2^{3p}-1=8^{p}-1

And by Fermat's little theorem q 2 q 1 1 q\mid 2^{q-1}-1 , since gcd ( q , 2 ) = 1 \gcd(q,2)=1 since q > 7 q>7 . Same for q 8 q 1 1 q\mid 8^{q-1}-1 .

Then q 2 gcd ( 3 p , q 1 ) 1 q\mid 2^{\gcd(3p,q-1)}-1 and q 8 gcd ( p , q 1 ) 1 q\mid 8^{\gcd(p,q-1)}-1 .

Assume for contradiction gcd ( p , q 1 ) = 1 \gcd(p,q-1)=1 . Then q 8 1 1 = 7 q\mid 8^1-1=7 , impossible since q > 7 q>7 . Therefore gcd ( p , q 1 ) = p \gcd(p,q-1)=p .

Assume for contradiction gcd ( 3 p , q 1 ) = p \gcd(3p,q-1)=p . Then q 2 p 1 q\mid 2^p-1 . But q 2 2 p + 2 p + 1 = ( 2 p 1 ) 2 + 3 ( 2 p 1 ) + 3 q 3 q\mid 2^{2p}+2^p+1=\left(2^p-1\right)^2+3\cdot \left(2^p-1\right)+3\iff q\mid 3 , impossible since q > 7 q>7 . Therefore gcd ( 3 p , q 1 ) = 3 p \gcd(3p,q-1)=3p , so q 1 = 3 p k q-1=3pk for some k Z 1 k\in\Bbb Z_{\ge 1} . q 1 q-1 is even and 3 p 3p is odd, so q 1 = 6 p m q-1=6pm for some m Z 1 m\in\Bbb Z_{\ge 1} .

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