A113883 Indices of semiprime Bell numbers A000110.

4, 6, 16, 31, 33, 49, 84

Offset

1,1

Comments

Semiprime analog of A051130 (indices of prime Bell numbers). These indices of semiprime Bell numbers include all values through B(120), which has 146 digits and at least 5 prime factors, the smallest being 71.

Links

Table of n, a(n) for n=1..7.

John Sokol, The First 1000 Bell Numbers.

Formula

n such that A000110(n) is semiprime. n such that A000110(n) is in A001358.

Example

a(1) = 4 because B(4) = 15 = 3 * 5.
a(2) = 6 because B(6) = 203 = 7 * 29.
a(3) = 16 because B(16) = 10480142147 = 241 * 43486067.
a(4) = 31 because B(31) = 10293358946226376485095653
= 11 * 935759904202397862281423.
a(5) = 33 because B(33) = 1629595892846007606764728147
= 5694673 * 286161451736738458339.
a(6) = 49 because B(49) =
10726137154573358400342215518590002633917247281
= 7615441337805454611187 *
1408472165798904899327563.
a(7) = 84 because B(84) is a 93-digit semiprime, whose smaller prime factor is 8429925224798761223.

Crossrefs

Cf. A000110, A001358, A113015.

Sequence in context: A081487 A099430 A183370 * A036748 A162485 A188466

Adjacent sequences: A113880 A113881 A113882 * A113884 A113885 A113886

Keyword

easy,nonn

Author

Jonathan Vos Post, Jan 27 2006

Status

approved