OFFSET
0,13
COMMENTS
LINKS
Emma Y. Jin, Jing Qin and Christian M. Reidys, Combinatorics of RNA structures with pseudoknots, arXiv:0704.2518 [math.CO], 2007.
Emma Y. Jin, Jing Qin and Christian M. Reidys, Combinatorics of RNA structures with pseudoknots, Bulletin of Mathematical Biology Vol. 70 (2008) pp. 45-67.
FORMULA
T(n,k) = Sum_{j=0..(n-k)/2} (-1)^j*binomial(n-j,j)*binomial(n-2j,k)*(c((n-k)/2-2j)*c((n-k)/2-j+2) - c((n-k)/2 - j + 1)^2), where c(n)=A000108(n) are the Catalan numbers (see Corollary 2 in the Jin et al. reference).
EXAMPLE
T(4,2)=3 because we have AIAI, IAIA, AIIA, where in each structure the two A's are joined by an arc and the two I's are isolated vertices.
T(4,4)=1 because we have IIII.
T(4,0)=1 because we have ABAB, where the two A's are joined by an arc and the two B's are joined by an arc.
Triangle starts:
1;
0, 1;
0, 0, 1;
0, 1, 0, 1;
1, 0, 3, 0, 1;
0, 6, 0, 6, 0, 1;
4, 0, 21, 0, 10, 0, 1.
MAPLE
c := proc (n) options operator, arrow: binomial(2*n, n)/(n+1) end proc: T := proc (n, l) if `mod`(n-l, 2) = 0 then sum((-1)^b*binomial(n-b, b)*binomial(n-2*b, l)*(c((1/2)*n-(1/2)*l-b)*c((1/2)*n-(1/2)*l-b+2)-c((1/2)*n-(1/2)*l-b+1)^2), b = 0 .. (1/2)*n-(1/2)*l) else 0 end if end proc: for n from 0 to 12 do seq(T(n, k), k = 0 .. n) end do; # yields sequence in triangular form
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Emeric Deutsch, Apr 24 2011
STATUS
approved