OFFSET
1,2
LINKS
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).
FORMULA
a(n) = (n^4 + 8*n^3 + 11*n^2 - 20*n + 12)/12.
a(n) = [x^n] (-3*x^4 + 11*x^3 - 13*x^2 + 4*x - 1)/(x - 1)^5. - Peter Luschny, Apr 10 2023
EXAMPLE
The a(2) = 8 shapes are 211, 221, 222, 221/1, 222/1, 222/11, 221/11, 211/1.
MAPLE
a := proc(n) m := n*(n + 4); m*(m - 5) / 12 + 1 end:
seq(a(n), n = 1..43); # Peter Luschny, Apr 10 2023
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Richard Stanley, Apr 09 2023
STATUS
approved