%I A001229
%S A001229 1,2,8,12,128,240,720,6912,32768,142560,712800,1140480,1190400,3345408,
%T A001229 3571200,5702400,14859936,29719872,50319360,118879488,2147483648,
%U A001229 3889036800,4389396480,21946982400,47416320000,92177326080,133145026560
%N A001229 phi(sigma(n)) = n.
%C A001229 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
%D A001229 Alaoglu and Erdos: A conjecture...., Bull. Amer. Math. Soc. 50 (1944), 
               881-882
%D A001229 J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 128, p. 44, Ellipses, 
               Paris 2008.
%D A001229 J.-M. De Koninck & A. Mercier, 1001 Problemes en Theorie Classique Des 
               Nombres, Problem 702 pp. 92; 300-1, Ellipses Paris 2004.
%D A001229 R. K. Guy, Unsolved Problems in Number Theory, B42.
%H A001229 Fred W. Helenius (fredh(AT)ix.netcom.com), <a href="http://www.netcom.com/
               ~fredh/phisigma/pslist.html">365 solutions</a>
%H A001229 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               TotientFunction.html">Link to a section of The World of Mathematics.</
               a>
%H A001229 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               TotientFunction.html">Totient Function</a>
%Y A001229 Cf. A000010.
%Y A001229 Sequence in context: A013190 A126192 A066471 this_sequence A120000 A067678 
               A134905
%Y A001229 Adjacent sequences: A001226 A001227 A001228 this_sequence A001230 A001231 
               A001232
%K A001229 nonn
%O A001229 1,2
%A A001229 N. J. A. Sloane (njas(AT)research.att.com).
%E A001229 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.