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A127725
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Numbers that are 2-imperfect.
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9
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OFFSET
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1,1
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COMMENTS
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This sequence also contains n = 3074457344902430720 = 2^31*5*17*257*65537, which has the product of four Fermat primes (A019434). For this n, 3*n is a 3-imperfect number (A127726). - T. D. Noe, Apr 03 2009
62549517598720 is also a term (see the "43 terms > 2*10^11" link by Donovan Johnson in A127724). - Michel Marcus, Nov 05 2017
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LINKS
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EXAMPLE
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40 = 2^3 * 5, (8 - 4 + 2 - 1)(5 - 1) = 20 = 40 / 2, so 40 is in the sequence. - Jud McCranie, Aug 17 2019
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MATHEMATICA
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okQ[n_] := 2 Sum[d*(-1)^PrimeOmega[n/d], {d, Divisors[n]}] == n;
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PROG
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(PARI) isok(n) = 2*sumdiv(n, d, d*(-1)^bigomega(n/d)) == n; \\ Michel Marcus, Oct 28 2017
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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