OFFSET
1,2
COMMENTS
For n=1, an equilateral triangle, there is no diagonal, and thus the polygon itself is the only triangle.
LINKS
Index entries for linear recurrences with constant coefficients, signature (7,-21,35,-35,21,-7,1).
FORMULA
a(n) = n*(2*n+1)*(2*n-1)*(2*n^3+21*n^2-2*n+9)/90.
G.f.: x*(x^5+20*x^4+7*x^3-63*x^2-28*x-1)/(x-1)^7. - Alois P. Heinz, Feb 11 2021
E.g.f.: exp(x)*x*(90 + 1485*x + 2775*x^2 + 1350*x^3 + 204*x^4 + 8*x^5)/90. - Stefano Spezia, Feb 12 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Edward Porcella, Feb 11 2021
STATUS
approved