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A133107
Number of Ferrers diagrams with a single strictly smaller Ferrers puncture with the same orientation removed from the top with half-perimeter = n.
2
1, 7, 32, 121, 410, 1294, 3888, 11273, 31826, 88041, 239734, 644758, 1717191, 4538129, 11919760, 31156313, 81125827, 210604604, 545462798, 1410226551, 3641097828, 9391872711, 24208902420, 62373915102, 160663604377
OFFSET
6,2
LINKS
Arvind Ayyer, Sep 11 2007, Table of n, a(n) for n = 6..76
FORMULA
G.f.: x^2*(-1 + 3*x - x^2 + (5*x^4 - 6*x^3 + 11*x^2 - 6*x + 1 + 4*x^6 - 12*x^5)^(1/2))/(2*(x^2 - 3*x + 1)*(1-2*x)^2)
EXAMPLE
The sequence starts with n=6 because the smallest such object whose illustration is below has a perimeter of 12. (1 denotes cell inside the Ferrers diagram.)
1 1
111
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Arvind Ayyer, Sep 11 2007
STATUS
approved