OFFSET
1,4
LINKS
Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8.
Index entries for linear recurrences with constant coefficients, signature (5, -7, 4).
FORMULA
G.f.: x^3 (1-x)^2/(1-5x+7x^2-4x^3).
a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6.
a(n) = a(n-1) + A001169(n-2) for n >= 3.
MATHEMATICA
a[ n_ ] := a[ n ]=If[ n<6, {0, 0, 1, 3, 9}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Dean Hickerson, Aug 10 1999
STATUS
approved