A131960 a(n) = A000043(n) * A000668(n).

6, 21, 155, 889, 106483, 2228207, 9961453, 66571993057, 140656423562035331011, 55088331748199422233011027879, 17361742620725829882898847100829589, 21607930299479592429924287571917281427329

Offset

1,1

Comments

Note that a(3)=155 and a(4)=889 also belong to A119691.

Old name was: Composite numbers such that the first factor is the relevant exponent of Mersenne prime (the second factor).

a(n) is the smallest k > 0 such that A000668(n)^2 divides 2^k-1. Then lpf(2^k-1) = A000668(n), where lpf(m) = A020639(m). - Thomas Ordowski, Feb 03 2019

Links

Table of n, a(n) for n=1..12.

Formula

a(n) = A000043(n)*A000668(n). - R. J. Mathar, Oct 18 2007

a(n) = A002326((A000668(n)^2 - 1)/2). - Thomas Ordowski, Feb 03 2019

Prog

(PARI) expm = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127]; vector(#expm, i, expm[i]*(2^expm[i]-1)) \\ where expm comes from A000043; Michel Marcus, Feb 07 2019

Crossrefs

Cf. A000043, A000668, A002326, A020639, A119691.

Sequence in context: A304264 A210443 A179768 * A372137 A244299 A143049

Adjacent sequences: A131957 A131958 A131959 * A131961 A131962 A131963

Keyword

nonn,less

Author

A.K. Devaraj, Aug 02 2007, Aug 06 2007

Extensions

Terms corrected, edited, and new name by Michel Marcus, Apr 30 2013

a(12) from Michel Marcus, Feb 07 2019

Status

approved