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A001229
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phi(sigma(n)) = n.
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4
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 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 (mymontain(AT)yahoo.com), Oct 08 2004
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REFERENCES
| Alaoglu and Erdos: A conjecture...., Bull. Amer. Math. Soc. 50 (1944), 881-882
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
| Fred W. Helenius (fredh(AT)ix.netcom.com), 365 solutions
Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
Eric Weisstein's World of Mathematics, Totient Function
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CROSSREFS
| Cf. A000010.
Sequence in context: A176968 A126192 A066471 * A120000 A067678 A134905
Adjacent sequences: A001226 A001227 A001228 * A001230 A001231 A001232
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| More terms from David W. Wilson (davidwwilson(AT)comcast.net) Aug 15 1996 (search was complete only though a(19) = 50319360). Jud McCranie (JudMcCranie(AT)ugaalum.uga.edu) reports Jun 15 1998 that the terms through a(24) are certain.
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