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 A091965 Triangle read by rows: T(n,k) = number of lattice paths from (0,0) to (n,k) that do not go below the line y=0 and consist of steps U=(1,1), D=(1,-1) and three types of steps H=(1,0) (left factors of 3-Motzkin steps). 33
 1, 3, 1, 10, 6, 1, 36, 29, 9, 1, 137, 132, 57, 12, 1, 543, 590, 315, 94, 15, 1, 2219, 2628, 1629, 612, 140, 18, 1, 9285, 11732, 8127, 3605, 1050, 195, 21, 1, 39587, 52608, 39718, 19992, 6950, 1656, 259, 24, 1, 171369, 237129, 191754, 106644, 42498, 12177, 2457 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS T(n,0) = A002212(n+1), T(n,1) = A045445(n+1); row sums give A026378. The inverse is A207815. - Gary W. Adamson, Dec 17 2006 [corrected by Philippe Deléham, Feb 22 2012] Reversal of A084536. - Philippe Deléham, Mar 23 2007 Triangle T(n,k), 0 <= k <= n, read by rows given by T(0,0)=1, T(n,k)=0 if k < 0 or if k > n, T(n,0) = 3*T(n-1,0) + T(n-1,1), T(n,k) = T(n-1,k-1) + 3*T(n-1,k) + T(n-1,k+1) for k >= 1. - Philippe Deléham, Mar 27 2007 This triangle belongs to the family of triangles defined by T(0,0)=1, T(n,k)=0 if k < 0 or if k > n, T(n,0) = x*T(n-1,0) + T(n-1,1), T(n,k) = T(n-1,k-1) + y*T(n-1,k) + T(n-1,k+1) for k >= 1. Other triangles arise by choosing different values for (x,y): (0,0) -> A053121; (0,1) -> A089942; (0,2) -> A126093; (0,3) -> A126970; (1,0)-> A061554; (1,1) -> A064189; (1,2) -> A039599; (1,3) -> A110877; (1,4) -> A124576; (2,0) -> A126075; (2,1) -> A038622; (2,2) -> A039598; (2,3) -> A124733; (2,4) -> A124575; (3,0) -> A126953; (3,1) -> A126954; (3,2) -> A111418; (3,3) -> A091965; (3,4) -> A124574; (4,3) -> A126791; (4,4) -> A052179; (4,5) -> A126331; (5,5) -> A125906. - Philippe Deléham, Sep 25 2007 5^n = (n-th row terms) dot (first n+1 terms in (1,2,3,...)). Example for row 4: 5^4 = 625 = (137, 132, 57, 12, 1) dot (1, 2, 3, 4, 5) = (137 + 264 + 171 + 48 + 5) = 625. - Gary W. Adamson, Jun 15 2011 Riordan array ((1-3*x-sqrt(1-6*x+5*x^2))/(2*x^2), (1-3*x-sqrt(1-6*x+5*x^2))/(2*x)). - Philippe Deléham, Feb 19 2012 REFERENCES A. Nkwanta, Lattice paths and RNA secondary structures, DIMACS Series in Discrete Math. and Theoretical Computer Science, 34, 1997, 137-147. LINKS Vincenzo Librandi, Rows n = 0..100, flattened FORMULA G.f.: G = 2/(1 - 3*z - 2*t*z + sqrt(1-6*z+5*z^2)). Alternatively, G = M/(1 - t*z*M), where M = 1 + 3*z*M + z^2*M^2. Sum_{k>=0} T(m, k)*T(n, k) = T(m+n, 0) = A002212(m+n+1). - Philippe Deléham, Sep 14 2005 The triangle may also be generated from M^n * [1,0,0,0,...], where M = an infinite tridiagonal matrix with 1's in the super and subdiagonals and [3,3,3,...] in the main diagonal. - Gary W. Adamson, Dec 17 2006 Sum_{k=0..n} T(n,k)*(k+1) = 5^n. - Philippe Deléham, Mar 27 2007 Sum_{k=0..n} T(n,k)*x^k = A117641(n), A033321(n), A007317(n), A002212(n+1), A026378(n+1) for x = -3, -2, -1, 0, 1 respectively. - Philippe Deléham, Nov 28 2009 T(n,k) = (k+1)*Sum_{m=k..n} binomial(2*(m+1),m-k)*binomial(n,m)/(m+1). - Vladimir Kruchinin, Oct 08 2011 EXAMPLE Triangle begins:      1;      3,    1;     10,    6,    1;     36,   29,    9,    1;    137,  132,   57,   12,    1;    543,  590,  315,   94,   15,    1;   2219, 2628, 1629,  612,  140,   18,    1; T(3,1)=29 because we have UDU, UUD, 9 HHU paths, 9 HUH paths and 9 UHH paths. Production matrix begins   3, 1;   1, 3, 1;   0, 1, 3, 1;   0, 0, 1, 3, 1;   0, 0, 0, 1, 3, 1;   0, 0, 0, 0, 1, 3, 1;   0, 0, 0, 0, 0, 1, 3, 1;   0, 0, 0, 0, 0, 0, 1, 3, 1;   0, 0, 0, 0, 0, 0, 0, 1, 3, 1;   0, 0, 0, 0, 0, 0, 0, 0, 1, 3, 1; - Philippe Deléham, Nov 07 2011 MATHEMATICA nmax = 9; t[n_, k_] := ((k+1)*n!*Hypergeometric2F1[k+3/2, k-n, 2k+3, -4]) / ((k+1)!*(n-k)!); Flatten[ Table[ t[n, k], {n, 0, nmax}, {k, 0, n}]] (* Jean-François Alcover, Nov 14 2011, after Vladimir Kruchinin *) T[0, 0, x_, y_] := 1; T[n_, 0, x_, y_] := x*T[n - 1, 0, x, y] + T[n - 1, 1, x, y]; T[n_, k_, x_, y_] := T[n, k, x, y] = If[k < 0 || k > n, 0, T[n - 1, k - 1, x, y] + y*T[n - 1, k, x, y] + T[n - 1, k + 1, x, y]]; Table[T[n, k, 3, 3], {n, 0, 10}, {k, 0, n}] // Flatten (* G. C. Greubel, May 22 2017 *) PROG (Maxima) T(n, k):=(k+1)*sum((binomial(2*(m+1), m-k)*binomial(n, m))/(m+1), m, k, n); / Vladimir Kruchinin, Oct 08 2011 */ (Sage) @CachedFunction def A091965(n, k):     if n==0 and k==0: return 1     if k<0 or k>n: return 0     if k==0: return 3*A091965(n-1, 0)+A091965(n-1, 1)     return A091965(n-1, k-1)+3*A091965(n-1, k)+A091965(n-1, k+1) for n in (0..7):     [A091965(n, k) for k in (0..n)] # Peter Luschny, Nov 05 2012 CROSSREFS Cf. A002212, A045445, A026378. Cf. A123965. Sequence in context: A134283 A035324 A171814 * A171568 A107056 A116384 Adjacent sequences:  A091962 A091963 A091964 * A091966 A091967 A091968 KEYWORD nonn,tabl AUTHOR Emeric Deutsch, Mar 13 2004 STATUS approved

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Last modified February 25 04:05 EST 2018. Contains 299630 sequences. (Running on oeis4.)