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A113883 Indices of semiprime Bell numbers A000110. 2
4, 6, 16, 31, 33, 49, 84 (list; graph; refs; listen; history; text; internal format)
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
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
Sequence in context: A081487 A099430 A183370 * A036748 A162485 A188466
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Jan 27 2006
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)