OFFSET
1,5
COMMENTS
Row sums yield the RNA secondary structure numbers (A004148).
Row n has ceiling(n/2) terms.
LINKS
I. L. Hofacker, P. Schuster and P. F. Stadler, Combinatorics of RNA secondary structures, Discrete Appl. Math., 88, 1998, 207-237.
P. R. Stein and M. S. Waterman, On some new sequences generalizing the Catalan and Motzkin numbers, Discrete Math., 26 (1979), 261-272.
M. Vauchassade de Chaumont and G. Viennot, Polynômes orthogonaux et problèmes d'énumération en biologie moléculaire, Sem. Loth. Comb. B08l (1984) 79-86.
FORMULA
T(n,k) = k*Sum_{j=ceiling((n-2k+2)/2)..n-2k+1} (1/j)*binomial(j, n-2k+1-j)*binomial(j+k-1, n-k+1-j) if 2k < n+1 and T(n,k)=1 if 2k = n+1.
G.f.: t*z*g/(1-t*z^2*g), where g = (1 - z + z^2 - sqrt(1 - 2*z - z^2 - 2*z^3 + z^4))/(2z^2) is the g.f. for the RNA secondary structure numbers (A004148).
EXAMPLE
Triangle starts:
1;
1;
1, 1;
2, 2;
4, 3, 1;
8, 6, 3;
17, 13, 6, 1;
T(5,2)=3 because we have UHDHH, UHHDH and UHHHD, where U=(1,1), H=(1,0) and D=(1,-1); these are the only peakless Motzkin paths of length 5 in which the second step is the first occurrence of H.
MAPLE
T:=proc(n, k) if 2*k-1=n then 1 else k*sum(binomial(j, n-2*k+1-j)*binomial(j+k-1, n-k+1-j)/j, j=ceil((n-2*k+2)/2)..n-2*k+1) fi end: seq(seq(T(n, k), k=1..ceil(n/2)), n=0..16); # yields the sequence in linear form T:=proc(n, k) if 2*k-1=n then 1 else k*sum(binomial(j, n-2*k+1-j)*binomial(j+k-1, n-k+1-j)/j, j=ceil((n-2*k+2)/2)..n-2*k+1) fi end: matrix(10, 10, T); # yields the sequence in triangular form
MATHEMATICA
T[n_, k_] := Which[2k < n+1, k*Sum[(1/j)*Binomial[j, n-2k+1-j]* Binomial[j+k-1, n-k+1-j], {j, Ceiling[(n-2k+2)/2], n-2k+1}], 2k == n+1, 1, True, 0];
Table[T[n, k], {n, 0, 16}, {k, 1, Ceiling[n/2]}] // Flatten (* Jean-François Alcover, Jul 18 2024 *)
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Emeric Deutsch, Sep 13 2004
STATUS
approved