OFFSET
1,6
COMMENTS
Row n has 2n-1 terms. Column 2 yields the Catalan numbers (A000108). T(n,3)=2T(n,2) (n>=3). Sum(kT(n,k),k=2..2n)=2[Catalan(n+1)-Catalan(n)] (A071721). The trivariate g.f., with z marking semilength, t marking height of the first peak and s marking height of the last peak, is G = (1-tzC-szC+tsz^2*C^2+tsz^2*C)/[(1-tzC)(1-szC)(1-tsz)]-1.
LINKS
Krishna Menon and Anurag Singh, Grassmannian permutations avoiding identity, arXiv:2212.13794 [math.CO], 2022.
FORMULA
G.f.: (1-2tzC+t^2*z^2*C^2+t^2*z^2*C)/[(1-tzC)^2*(1-t^2*z)]-1, where C=[1-sqrt(1-4z)]/(2z) is the Catalan function.
EXAMPLE
T(5,6)=4 because we have UUDUUUDDDD, UUUDUDUDDD, UUUDDUUDDD and UUUUDDDUDD, where U=(1,1), D=(1,-1).
Triangle starts:
1;
1,0,1;
1,2,1,0,1;
2,4,4,2,1,0,1;
5,10,11,8,4,2,1,0,1;
...
MAPLE
C:=(1-sqrt(1-4*z))/2/z: g:=(1-2*t*z*C+t^2*z^2*C^2+t^2*z^2*C)/(1-t*z*C)^2/(1-t^2*z)-1: gser:=simplify(series(g, z=0, 12)): for n from 1 to 10 do P[n]:=coeff(gser, z^n) od: for n from 1 to 10 do seq(coeff(P[n], t^j), j=2..2*n) od; # yields sequence in triangular form
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Emeric Deutsch, Dec 02 2005
STATUS
approved