A359666 Integers k such that sigma(k) <= sigma(k+1) <= sigma(k+2) <= sigma(k+3), where sigma is the sum of divisors.

1, 13, 61, 73, 133, 145, 193, 205, 253, 397, 457, 481, 493, 553, 565, 613, 625, 661, 673, 733, 757, 793, 817, 853, 913, 973, 997, 1033, 1093, 1213, 1237, 1285, 1321, 1333, 1453, 1513, 1537, 1633, 1645, 1657, 1681, 1813, 1825, 1873, 1933, 2077, 2113, 2173, 2233, 2245, 2293, 2413, 2497

Offset

1,2

Links

Seiichi Manyama, Table of n, a(n) for n = 1..10000

Example

73 is a term because sigma(73)=74 <= sigma(74)=114 <= sigma(75)=124 <= sigma(76)=140.

Mathematica

Position[OrderedQ /@ Partition[DivisorSigma[1, Range[2500]], 4, 1], True] // Flatten (* Amiram Eldar, Feb 28 2023 *)

Prog

(PARI) isok(n)=sigma(n)<=sigma(n+1) && sigma(n+1)<=sigma(n+2) && sigma(n+2)<=sigma(n+3)

Crossrefs

Cf. A000203, A053224, A323726, A364662.

Sequence in context: A146764 A336794 A145474 * A217606 A002647 A306127

Adjacent sequences: A359663 A359664 A359665 * A359667 A359668 A359669

Keyword

nonn,easy

Author

Alexandru Petrescu, Feb 28 2023

Status

approved