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A001229
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Numbers n such that phi(sigma(n)) = n.
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12
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1, 2, 8, 12, 128, 240, 720, 6912, 32768, 142560, 712800, 1140480, 1190400, 3345408, 3571200, 5702400, 14859936, 29719872, 50319360, 118879488, 2147483648, 3889036800, 4389396480, 21946982400, 47416320000, 92177326080, 133145026560, 331914240000
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OFFSET
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1,2
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COMMENTS
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For n=0,1,2,3,4 & 5 2^(2^n-1) is in the sequence because 2^2^n+1 is prime for n=0,1,2,3 & 4 (Fermat primes). - Farideh Firoozbakht, Oct 08 2004
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REFERENCES
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J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 128, p. 44, Ellipses, Paris 2008.
J.-M. De Koninck & A. Mercier, 1001 Problemes en Theorie Classique Des Nombres, Problem 702 pp. 92; 300-1, Ellipses Paris 2004.
R. K. Guy, Unsolved Problems in Number Theory, B42.
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LINKS
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FORMULA
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MATHEMATICA
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Select[Range[10000], EulerPhi[DivisorSigma[1, #]] == # &] (* T. D. Noe, Jun 26 2012 *)
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PROG
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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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More terms from David W. Wilson, Aug 15 1996 (search was complete only through a(19) = 50319360).
Jud McCranie reports Jun 15 1998 that the terms through a(24) are certain.
a(28) added. Verified sequence is complete through a(28) by Donovan Johnson, Jun 30 2012
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STATUS
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approved
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