A067828 - OEIS

A067828

Odd numbers k such that sigma(k+1) < sigma(k).

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

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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

Amiram Eldar, Table of n, a(n) for n = 1..10000

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

Cf. A000203, A053224, A067825.

Sequence in context: A247887 A369185 A063069 * A144552 A068355 A331190

Adjacent sequences: A067825 A067826 A067827 * A067829 A067830 A067831

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Feb 08 2002

STATUS

approved