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