A113883 Indices of semiprime Bell numbers A000110.

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

Offset: 1

Author

Jonathan Vos Post, Jan 27 2006

Keywords

• easy
• nonn

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.

Examples

			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.
		

Links

• John Sokol, The First 1000 Bell Numbers.

Crossrefs

Cf. A000110, A001358, A113015.

Formula

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