A128728 - OEIS

%I #19 Jul 06 2023 01:56:53

%S 1,1,2,1,6,4,20,16,71,64,2,262,261,20,994,1084,141,3852,4572,854,7,

%T 15183,19520,4772,112,60686,84139,25416,1128,245412,365404,131270,

%U 9120,30,1002344,1596420,664004,64790,660,4129012,7008544,3309336,422928

%N Triangle read by rows: T(n,k) is the number of skew Dyck paths of semilength n and having k UDL's (n >= 0; 0 <= k <= floor((n+1)/2)).

%C A skew Dyck path is a path in the first quadrant which begins at the origin, ends on the x-axis, consists of steps U=(1,1)(up), D=(1,-1)(down) and L=(-1,-1)(left) so that up and left steps do not overlap. The length of a path is defined to be the number of steps in it.

%C Row n has 1 + floor((n+1)/3) terms.

%C Row sums yield A002212.

%C T(n,0) = A128729(n).

%C Sum_{k>=0} k*T(n,k) = A128730(n).

%C Apparently, T(3k-1,k) = binomial(3k-1,k)/(3k-1) = A006013(k-1).

%H E. Deutsch, E. Munarini, and S. Rinaldi, <a href="http://dx.doi.org/10.1016/j.jspi.2010.01.015">Skew Dyck paths</a>, J. Stat. Plann. Infer. 140 (8) (2010) 2191-2203.

%H Helmut Prodinger, <a href="https://arxiv.org/abs/2203.10516">Skew Dyck paths without up-down-left</a>, arXiv:2203.10516 [math.CO], 2022.

%H Yuxuan Zhang and Yan Zhuang, <a href="https://arxiv.org/abs/2306.15778">A subfamily of skew Dyck paths related to k-ary trees</a>, arXiv:2306.15778 [math.CO], 2023.

%F G.f.: G = G(t,z) satisfies z^2*G^3 - z(2-z)G^2 + (1-z^2)G - 1 + z + z^2 - tz^2 = 0.

%e T(3,1)=4 because we have UDUUDL, UUUDLD, UUDUDL and UUUDLL.

%e Triangle starts:

%e     1;

%e     1;

%e     2,   1;

%e     6,   4;

%e    20,  16;

%e    71,  64,   2;

%e   262, 261,  20;

%p eq:=z^2*G^3-z*(2-z)*G^2+(1-z^2)*G-1+z+z^2-t*z^2=0: G:=RootOf(eq,G): Gser:=simplify(series(G,z=0,17)): for n from 0 to 14 do P[n]:=sort(coeff(Gser,z,n)) od: for n from 0 to 14 do seq(coeff(P[n],t,j),j=0..floor((n+1)/3)) od; # yields sequence in triangular form

%Y Cf. A002212, A006013, A128729, A128730.

%K nonn,tabf

%O 0,3

%A _Emeric Deutsch_, Mar 31 2007