A069155 Nonprime numbers k such that (k+1)*Sum_{d|k} 1/(d+1) is an integer.

1, 10, 55, 161, 209, 551, 649, 1079, 1189, 3401, 6049, 6319, 9701, 12151, 14279, 14849, 25271, 34399, 40753, 51841, 64769, 85919, 150281, 158801, 193159, 194399, 246401, 286579, 456191, 514999, 546209, 563471, 647351, 920639, 1156609, 1211401, 1721249, 1934711

Offset

1,2

Comments

Or, nonprime numbers k such that Sum_{d|k} C(k,d+1)/C(k,d) is an integer.

Links

Amiram Eldar, Table of n, a(n) for n = 1..175

Mathematica

Select[Range[2, 1250000], ! PrimeQ[ # ] && IntegerQ[( # + 1)*Apply[Plus, 1/(Divisors[ # ] + 1)]] & ]

Prog

(PARI) isok(k) = !isprime(k) && denominator((k+1) * sumdiv(k, d, 1/(d+1))) == 1; \\ Amiram Eldar, Apr 19 2025

Crossrefs

Sequence in context: A119334 A321780 A074977 * A107352 A127761 A337898

Adjacent sequences: A069152 A069153 A069154 * A069156 A069157 A069158

Keyword

nonn

Author

Benoit Cloitre, Apr 08 2002

Extensions

Edited and extended by Robert G. Wilson v, Apr 30 2002

Entry revised by N. J. A. Sloane Apr 09 2007 following suggestions by Ryan Propper and Emeric Deutsch.

a(36)-a(38) from Amiram Eldar, Apr 19 2025

Status

approved