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
Eric Weisstein's World of Mathematics, Sociable Numbers
MATHEMATICA
Select[ Range@8323071, DivisorSigma[1, DivisorSigma[1, # ]/2] == 2# &] (* Robert G. Wilson v *)
CROSSREFS
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