A unitary perfect number is a positive integer 
which is the sum of its unitary
divisors with the exception of 
itself. There are no odd unitary
perfect numbers (Subbarao and Warren 1966, pp. 148-149), and it has been conjectured
that there are only a finite number of even
ones. The five known examples are 6, 60, 90, 87360, 146361946186458562560000 (OEIS
A002827).
Unitary Perfect Number
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References
Guy, R. K. "Unitary Perfect Numbers." §B3 in Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 53-59, 1994.Sloane, N. J. A. Sequence A002827/M4268 in "The On-Line Encyclopedia of Integer Sequences."Subbarao, M. V. and Warren, L. J. "Unitary Perfect Numbers." Canad. Math. Bull. 9, 147-153, 1966. https://doi.org/10.4153/CMB-1966-018-4.Wall, C. R. "The Fifth Unitary Perfect Number." Canad. Math. Bull. 18, 115-122, 1975. https://doi.org/10.4153/CMB-1975-021-9.Wall, C. R. "On the Largest Odd Component of a Unitary Perfect Number." Fib. Quart. 25, 312-316, 1987.Referenced on Wolfram|Alpha
Unitary Perfect NumberCite this as:
Weisstein, Eric W. "Unitary Perfect Number." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/UnitaryPerfectNumber.html