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
FORMULA
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
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