# Math Help - Finding all primes P

1. ## Finding all primes P

Find all primes p such that:

8p^2 + 1 is also a prime,

Thanks

-Kate

2. Originally Posted by katie07

Find all primes p such that:

8p^2 + 1 is also a prime,

Thanks

-Kate
If p is odd then p = 1(mod 3) or p = 2(mod 3) in either case p^2=1(mod 3) so 8p^2+1 is always divisible by 3.

3. Originally Posted by ThePerfectHacker
If p is odd then p = 1(mod 3) or p = 2(mod 3) in either case p^2=1(mod 3) so 8p^2+1 is always divisible by 3.

So you leave the possibilities p=2 or p=3.

Now for p=2

8p^2+1 = 33

which is not prime.

For p=3

8p^2+1 = 73

which is prime.

RonL