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

Author

Alexandru Petrescu, Feb 28 2023

Keywords

• nonn
• easy

Examples

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

Links

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

Crossrefs

Cf. A000203, A053224, A323726, A364662.

Programs

Mathematica

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

PARI

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