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A064012
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usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448).
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3
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OFFSET
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1,1
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COMMENTS
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Is this sequence finite?
18 is the only known integer usigma(usigma(n)) = kn for some integer k >= 4 (no other one <= 2^30). - Tomohiro Yamada, Apr 22 2017
a(7) > 2.85*10^11, if it exists. The same bound holds also for any n > 18 such usigma(usigma(n)) = k*n for some integer k >= 4. - Giovanni Resta, Apr 10 2019
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LINKS
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EXAMPLE
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10 belongs to the sequence because usigma(usigma(10)) = 30 = 3*10.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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