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%I #11 Apr 19 2016 01:16:08
%S 6,12,14,28,48,62,112,124,160,189,192,254,448,496,508,1984,2032,8128,
%T 12288,16382,28672,32764,126976,131056,196608,262142,458752,520192,
%U 524224,524284,786432,1048574,1835008,2031616,2097136,2097148,8126464
%N Let S(n)=Sigma(n)/2. Numbers n such that S(S(n))=n, 1/2-Sociable number of order 1 or 2.
%C Almost all terms are of the form 2^m*M_k where M_k means the Mersenne prime 2^k-1. a(9)=2^5*5 and a(10)=3^3*7 are sporadic solutions. S(a(9))=a(10).
%C All numbers of the form (M_j+1)/2 M_k, where M_j and M_k are Mersenne primes, are in this sequence. - _Robert G. Wilson v_ and _T. D. Noe_, Jan 21 2006
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/SociableNumbers.html">Sociable Numbers</a>
%t Select[ Range@8323071, DivisorSigma[1, DivisorSigma[1, # ]/2] == 2# &] (* _Robert G. Wilson v_ *)
%K nonn
%O 1,1
%A _Yasutoshi Kohmoto_, Jan 21 2006
%E More terms from _Robert G. Wilson v_ and _T. D. Noe_, Jan 21 2006
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