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A067081
Numbers k such that sigma(k+1) = 2*sigma(k).
8
5, 125, 1253, 1673, 3127, 5191, 7615, 12035, 43817, 47795, 48559, 49955, 56975, 58373, 61721, 63545, 68033, 78395, 97411, 106001, 108473, 109939, 111451, 111683, 123497, 123797, 131659, 137081, 192065, 192279, 224561, 228377, 274273, 280313, 290417, 290719, 351815
OFFSET
1,1
COMMENTS
Sequence seems to consist only of odd values.
LINKS
MATHEMATICA
f[n_]:=DivisorSigma[1, n]; lst={}; Do[If[2*f[n]==f[n+1], AppendTo[lst, n]], {n, 9!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jul 22 2009 *)
Position[Partition[DivisorSigma[1, Range[300000]], 2, 1], _?(2#[[1]]==#[[2]]&), 1, Heads->False]//Flatten (* Harvey P. Dale, Oct 21 2024 *)
PROG
(PARI) isok(k) = { sigma(k+1) == 2*sigma(k) } \\ Harry J. Smith, May 08 2010
CROSSREFS
Sequence in context: A193612 A195949 A354654 * A195801 A154022 A013710
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Feb 18 2002
EXTENSIONS
Terms added by Harry J. Smith, May 08 2010
STATUS
approved