Find all two-digit prime number pairs $p$ and $q$, for which $pq+1$ is a perfect square.

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Find all two-digit prime number pairs p and q, for which $p\cdot q+1$ is a perfect square.

MY IDEAS

Because p and q are two digit prime numbers, clearly, they are odd numbers, so let $p=2a+1$ and $q=2b+1$ where $a, b$ are positive integers.

$$(2a+1)(2b+1)+1=(2k+1)^{2}+1$$ for some positive integer $k$.

Then we can apply the formula that says $(x+y)^2=x^2+y^2+2xy$.

But what should I do next? Hope one of you can help me! Thank you in advance!

Best Answer

Any pair of twin primes will work, because $(n+1)(n-1)=n^2-1$. Those are the only possible solutions because the problem is equivalent to saying $n^2-1=(n+1)(n-1)$ is a product of two primes, so those two primes must be $n-1$ and $n+1$.

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