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A113791 Let S(n)=Sigma(n)/2. Numbers n such that S(S(n))=n, 1/2-Sociable number of order 1 or 2. 1
6, 12, 14, 28, 48, 62, 112, 124, 160, 189, 192, 254, 448, 496, 508, 1984, 2032, 8128, 12288, 16382, 28672, 32764, 126976, 131056, 196608, 262142, 458752, 520192, 524224, 524284, 786432, 1048574, 1835008, 2031616, 2097136, 2097148, 8126464 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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).

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

LINKS

Table of n, a(n) for n=1..37.

Eric Weisstein's World of Mathematics, Sociable Numbers

MATHEMATICA

Select[ Range@8323071, DivisorSigma[1, DivisorSigma[1, # ]/2] == 2# &] (* Robert G. Wilson v *)

CROSSREFS

Sequence in context: A293907 A259397 A183029 * A292289 A281352 A135763

Adjacent sequences:  A113788 A113789 A113790 * A113792 A113793 A113794

KEYWORD

nonn

AUTHOR

Yasutoshi Kohmoto, Jan 21 2006

EXTENSIONS

More terms from Robert G. Wilson v and T. D. Noe, Jan 21 2006

STATUS

approved

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Last modified April 21 22:12 EDT 2019. Contains 322328 sequences. (Running on oeis4.)