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Search: id:A001229
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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; listen)
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OFFSET
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1,2
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COMMENT
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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 (f.firoozbakht(AT)math.ui.ac.ir), Oct 08 2004
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REFERENCES
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Alaoglu and Erdos: A conjecture...., Bull. Amer. Math. Soc. 50 (1944), 881-882
R. K. Guy, Unsolved Problems in Number Theory, B42.
J.-M. De Koninck & A. Mercier, 1001 Problemes en Theorie Classique Des Nombres, Problem 702 pp. 92; 300-1, Ellipses Paris 2004.
J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 128, p. 44, Ellipses, Paris 2008.
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LINKS
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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
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Cf. A000010.
Adjacent sequences: A001226 A001227 A001228 this_sequence A001230 A001231 A001232
Sequence in context: A013190 A126192 A066471 this_sequence A120000 A067678 A134905
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
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More terms from David W. Wilson (davidwwilson(AT)comcast.net) Aug 15 1996 (search was complete only though a(19) = 50319360). Jud McCranie (j.mccranie(AT)comcast.net) reports Jun 15 1998 that the terms through a(24) are certain.
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