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A067828
Odd numbers k such that sigma(k+1) < sigma(k).
3
45, 105, 117, 165, 225, 273, 297, 315, 345, 357, 405, 465, 513, 525, 561, 585, 621, 693, 705, 765, 777, 825, 837, 861, 885, 945, 1005, 1113, 1125, 1155, 1185, 1197, 1281, 1305, 1365, 1395, 1425, 1485, 1521, 1545, 1575, 1593, 1617, 1701, 1725, 1755, 1785, 1845
OFFSET
1,1
COMMENTS
Most terms are == 3 (mod 6), first term == 1 (mod 6) is a(130) = 5005. First term == 5 (mod 6) may be 247818996425. - Jianing Song, Apr 01 2018
LINKS
MATHEMATICA
Select[Range[1, 2000, 2], DivisorSigma[1, #] > DivisorSigma[1, # + 1] &] (* Amiram Eldar, Jun 06 2022 *)
Select[Flatten[Position[Partition[DivisorSigma[1, Range[2000]], 2, 1], _?(#[[2]]<#[[1]]&), 1, Heads-> False]], OddQ] (* Harvey P. Dale, Aug 02 2024 *)
PROG
(PARI) isok(n) = (n % 2) && (sigma(n+1) < sigma(n)); \\ Michel Marcus, Nov 23 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Feb 08 2002
STATUS
approved