OFFSET
1,2
COMMENTS
Fixed quasi-trominoes.
LINKS
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).
FORMULA
a(n)=n*(n-1)*(3*n^2-3*n-2)/2.
G.f.: 4*x^2*(1+7*x+x^2)/(1-x)^5. [Colin Barker, Apr 25 2012]
EXAMPLE
a(2)=4: the four rotations of the (connected) L tromino
MATHEMATICA
LinearRecurrence[{5, -10, 10, -5, 1}, {0, 4, 48, 204, 580}, 40] (* Harvey P. Dale, Aug 09 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
David Bevan, Jul 27 2009
STATUS
approved