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A072868
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Numbers n such that sigma(sigma(n)-n) = n.
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0
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OFFSET
| 1,1
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FORMULA
| It seems that a(n)=2^A000043(n) = M(n)+1 where M(n) is the n-th Mersenne prime
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MATHEMATICA
| f[n_]:=Plus@@Divisors[Plus@@Divisors[n]-n]==n; lst={}; Do[If[f[n], AppendTo[lst, n]], {n, 9!}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), May 14 2010]
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CROSSREFS
| Cf. A000043.
Sequence in context: A103970 A034785 A075398 * A098579 A032467 A009265
Adjacent sequences: A072865 A072866 A072867 * A072869 A072870 A072871
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KEYWORD
| easy,nonn
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AUTHOR
| Benoit Cloitre (benoit7848c(AT)orange.fr), Jul 27 2002
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