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 A001229 Numbers n such that phi(sigma(n)) = n. 10
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 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, Oct 08 2004 REFERENCES 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. LINKS Alaoglu and ErdÅ‘s, A conjecture in elementary number theory, Bull. Amer. Math. Soc. 50 (1944), pp. 881-882 Fred W. Helenius (fredh(AT)ix.netcom.com), 365 solutions T. Negadi, The genetic code invariance: when Euler and Fibonacci meet, arXiv preprint arXiv:1406.6092, 2014; Symmetry: Culture and Science, Vol. 25, No. 3, 261-278, 2014 Eric Weisstein's World of Mathematics, Totient Function. FORMULA phi(A018784), sorted. - David W. Wilson, Oct 18 2012 MATHEMATICA Select[Range[10000], EulerPhi[DivisorSigma[1, #]] == # &] (* T. D. Noe, Jun 26 2012 *) PROG (PARI) is(n)=eulerphi(sigma(n))==n \\ Charles R Greathouse IV, May 15 2013 CROSSREFS Cf. A000010, A018784, A135240. Sequence in context: A126192 A272720 A066471 * A298640 A120000 A067678 Adjacent sequences:  A001226 A001227 A001228 * A001230 A001231 A001232 KEYWORD nonn AUTHOR EXTENSIONS 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 STATUS approved

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Last modified February 22 09:43 EST 2019. Contains 320390 sequences. (Running on oeis4.)