Unitary Perfect Number
A unitary perfect number is a number 
which is the sum
of its unitary divisors with the exception of
itself. There are no odd
unitary perfect numbers, and it has been conjectured that there are only a finite
number of even ones. The first few are 6, 60, 90,
87360, 146361946186458562560000, ... (Sloane's A002827).
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basic definition of prime number