1,1

Semiprime analogue of A051130 (indices of prime Bell numbers) = 2, 3, 7, 13, 42, 55, 2841. These indices of semiprime Bell numbers include all values through B(90), which has 102 digits and at least 3 prime factors, the smallest being 2711.

John Sokol, The First 1000 Bell Numbers.

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

a(1) = 4 because B(4) = 15 = 2 * 3.

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.

Cf. A000110, A001358, A113015.

Sequence in context: A112576 A081487 A099430 this_sequence A036748 A162485 A076066

Adjacent sequences: A113880 A113881 A113882 this_sequence A113884 A113885 A113886

easy,nonn

Jonathan Vos Post (jvospost3(AT)gmail.com), Jan 27 2006