OFFSET
8,2
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 8..3296
FORMULA
a(n) = [(2*n^2-16*n+6)*a(n-1)+(4*n^2-68*n+240)*a(n-2)-(8*n^2-88*n+240)*a(n-3)]/(n^2-14*n+48) with a(6)=0, a(7)=0, a(8)=1.
G.f.: (1-(1-4*x^2)^(1/2))*x^6/(2*(2*x-1)^4).
EXAMPLE
The sequence starts with n=8 because the smallest such object whose cartoon is below has a perimeter of 16. (1 denotes cell outside the puncture and 2 denotes cell inside the puncture).
111
121
111
MATHEMATICA
Drop[CoefficientList[Series[(1-(1-4x^2)^(1/2))x^6/(2(2x-1)^4), {x, 0, 40}], x], 8] (* Harvey P. Dale, Sep 21 2024 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Arvind Ayyer, Sep 11 2007
STATUS
approved