Search: id:A121635
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%I A121635
%S A121635 1,2,8,42,264,1920,15840,146160,1491840,16692480,203212800,2674425600,
%T A121635 37841126400,572885913600,9240898867200,158228598528000,
%U A121635 2866422214656000,54775863926784000,1101208277385216000
%N A121635 Number of deco polyominoes of height n, having no 2-cell columns starting 
               at level 0. A deco polyomino is a directed column-convex polyomino 
               in which the height, measured along the diagonal, is attained only 
               in the last column.
%C A121635 a(n)=A121634(n,0).
%D A121635 E. Barcucci, A. Del Lungo and R. Pinzani, "Deco" polyominoes, permutations 
               and random generation, Theoretical Computer Science, 159, 1996, 29-42.
%F A121635 a(1)=1, a(n)=(n-2)!(n^2-3n+4)/2 for n>=2. a(1)=1, a(2)=1, a(n)=(n-2)[(n-2)! 
               + a(n-1)] for n>=3.
%e A121635 a(2)=1 because the deco polyominoes of height 2 are the horizontal and 
               vertical dominoes and the horizontal one has no 2-cell column starting 
               at level 0.
%p A121635 a:=n->(n^2-3*n+4)*(n-2)!/2: seq(a(n),n=2..23);
%Y A121635 Cf. A121634, A001710.
%Y A121635 Sequence in context: A130649 A054993 A005315 this_sequence A002874 A078592 
               A052646
%Y A121635 Adjacent sequences: A121632 A121633 A121634 this_sequence A121636 A121637 
               A121638
%K A121635 nonn
%O A121635 1,2
%A A121635 Emeric Deutsch (deutsch(AT)duke.poly.edu), Aug 13 2006

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