Determine the number of triplets of positive integers with such that the equation above is fulfilled.
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Let d = a b c − ( a + b + c ) = ( a b + b c + a c ) − 1 . Note d > 0 . Then ( a + i ) ( b + i ) ( c + i ) = d + d i so the complex argument of the product is 4 5 o , and the arguments of the terms are between 0 and 4 5 o , and the arguments of the terms are in decreasing order and sum to 4 5 o , so arg ( a + i ) is at least 1 5 o , so a ≤ tan ( 1 5 o ) 1 ≈ 3 . 7 3 2 , so a = 1 , 2 , 3 are the only possibilities.
a = 1 simplifies to b + c = 0 , which is impossible.
a = 2 simplifies to ( b − 3 ) ( c − 3 ) = 1 0 , which leads to ( 2 , 4 , 1 3 ) and ( 2 , 5 , 8 ) .
a = 3 simplifies to ( b − 2 ) ( c − 2 ) = 5 , which leads to ( 3 , 3 , 7 ) .
So there are a total of 3 solutions.
There is probably a way of getting the inequality without using complex analysis, but I think this way is pretty clean.