Calm down, it is easy

Find the sum of all prime numbers P P such that 4 P 2 + 1 4P^2+1 and 6 P 2 + 1 6P^2+1 are also primes.


The answer is 5.

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1 solution

Abdeslem Smahi
Aug 9, 2015

Let P P be a prime so:

Case 1 : If P = 5 P=5

4 P 2 + 1 = 101 4P^2+1=101 and 6 P 2 + 1 = 151 6P^2+1=151 which are both primes so 5 5 is a solution

Case 2 : If P 1 , 2 , 3 , 4 ( m o d 5 ) P \equiv 1,2,3,4 \pmod{5}

So P 2 1 , o r , 1 ( m o d 5 ) P^2 \equiv 1,or,-1 \pmod{5}

if P 2 1 ( m o d 5 ) P^2 \equiv 1 \pmod{5} so 4 P 2 + 1 0 ( m o d 5 ) 4P^2+1 \equiv 0 \pmod{5} which is not prime.

if P 2 1 ( m o d 5 ) P^2 \equiv -1 \pmod{5} so 6 P 2 + 1 0 ( m o d 5 ) 6P^2+1 \equiv 0 \pmod{5} which is not prime

So 5 5 is the only solution.

I had a nagging suspicion that this required quadratic residues. Very nice question!

Jake Lai - 5 years, 10 months ago

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Thank you :)

Abdeslem Smahi - 5 years, 10 months ago

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