OFFSET
1,3
COMMENTS
a(n) = A121637(n,0).
REFERENCES
E. Barcucci, A. Del Lungo and R. Pinzani, "Deco" polyominoes, permutations and random generation, Theoretical Computer Science, 159, 1996, 29-42.
FORMULA
D-finite with recurrence a(n)=(n-1)a(n-1)+a(n-3) for n>=3; a(1)=1, a(2)=1, a(3)=2.
EXAMPLE
a(2)=1 because the deco polyominoes of height 2 are the horizontal and vertical dominoes and only the horizontal one has no 2-cell column.
MAPLE
a[1]:=1: a[2]:=1: a[3]:=2: for n from 4 to 23 do a[n]:=(n-1)*a[n-1]+a[n-3] od: seq(a[n], n=1..23);
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Aug 14 2006
STATUS
approved