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.
FORMULA
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
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Apr 23 2010
STATUS
approved