Find all numbers $n$ so that there exists $n$ integers $a_1, a_2, ..., a_n$: $a_i \ge 2$ and $a_i\cdot a_j +1 (\forall i\not = j)$ are all perfect squares.
closed as off-topic by Martin R, Hippalectryon, user230715, Ofir Schnabel, David K Nov 5 '15 at 13:43
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