



1, 8, 44, 215, 1028, 5040, 25931, 141394, 818940, 5030489, 32670830, 223561960, 1606504121, 12086613500, 94951417768, 777028092855, 6609770035768, 58333927746852, 533203742855027, 5039919479205198, 49191925330095240
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OFFSET

2,2


COMMENTS

Partial sums of number of matroids of rank 2 on n labeled points. Partial sums of number of collections containing two or more nonempty subsets of {1,2,...n} that are pairwise disjoint. The subsequence of primes in this partial sum begins: 25931, 5030489.


LINKS

Table of n, a(n) for n=2..22.


FORMULA

a(n) = SUM[i=2..n] A058681(i) = SUM[i=2..n] (B(n+1)  2^n), where B = Bell numbers (A000110).


EXAMPLE

a(11) = 1 + 7 + 36 + 171 + 813 + 4012 + 20891 + 115463 + 677546 + 4211549 = 5030489 is prime.


MATHEMATICA

Accumulate[Table[BellB[n+1]2^n, {n, 2, 30}]] (* Harvey P. Dale, Feb 14 2012 *)


CROSSREFS

Cf. A058681.
Sequence in context: A292487 A125318 A000373 * A272154 A270935 A197213
Adjacent sequences: A176685 A176686 A176687 * A176689 A176690 A176691


KEYWORD

easy,nonn


AUTHOR

Jonathan Vos Post, Apr 23 2010


STATUS

approved



