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BdMO National 2013: Secondary 9, Higher Secondary 7
Posted: Fri Jan 10, 2014 1:40 am
by BdMO
If there exists a prime number $p$ such that $p+2q$ is prime for all positive integer $q$ smaller than $p$, then $p$ is called an "awesome prime". Find the largest "awesome prime" and prove that it is indeed the largest such prime.
Re: BdMO National 2013: Secondary 9, Higher Secondary 7
Posted: Sun Jan 12, 2014 5:50 pm
by Fatin Farhan
Re: BdMO National 2013: Secondary 9, Higher Secondary 7
Posted: Fri Feb 07, 2014 1:48 am
by shahriarabdullah
i p=2 then $2|p+2q$. So p cannot be 2.
Let p be the largest awesome prime.
The least value of q for any p is 1. So, we can write-
$ p+2\equiv1,2 (mod 3) $
now suppose,
$ p+2\equiv1 (mod 3) $
so, $p+4\equiv0 (mod 3) $
so, $2q=4$
and so, $q=2$. which is not possible.
So, for the largest awesome prime-
$ p+2\equiv2 (mod 3) $
or, $ p+6\equiv0 (mod 3) $
or, $ p\equiv0 (mod 3)$
Now we can say, p is a prime and divisible by 3. So the only value of $p=3$
And it's the largest awesome prime.
Re: BdMO National 2013: Secondary 9, Higher Secondary 7
Posted: Mon Apr 05, 2021 1:37 am
by Anindya Biswas
If $p>3$ then $p,p+2,p+4$ forms a complete set of residues mod $3$. Thus one of then is divisible by $3$. So, $3$ is the largest awesome prime.