cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-2 of 2 results.

A162326 Let a(0) = a(1) = 1, and n*a(n) = 2*(-7+5*n)*a(n-1) + 9*(2-n)*a(n-2) for n >= 2.

Original entry on oeis.org

1, 1, 3, 13, 71, 441, 2955, 20805, 151695, 1135345, 8671763, 67320573, 529626839, 4213228969, 33833367963, 273892683573, 2232832964895, 18314495896545, 151037687326755, 1251606057754605, 10416531069771111, 87029307323766681
Offset: 0

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Author

Georg Muntingh, Jul 01 2009

Keywords

Comments

Let y = y(x) be implicitly defined by g(x,y(x)) = 0, with dg/dy not identically zero. For n >= 1, the sequence a(n) is the number of terms in the expansion of the divided difference [x0,...,xn]y in terms of bivariate divided differences of g.
(1 + 3*x + 13*x^2 + 71*x^3 + ...) = (1 + 4*x + 20*x^2 + 116*x^3 + ...) * 1/(1 + x + 4*x^2 + 20*x^3 + 116*x^4 + ...); where A082298 = (1, 4, 20, 116, 740, ...). - Gary W. Adamson, Nov 17 2011
The shifted sequence 1,3,13,71,... is the binomial transform of A151374. - Georg Muntingh, Jul 19 2012
a(n+1) is the number of Schröder paths of semilength n in which the (2,0)-steps come in 3 colors and with no peaks at level 1. - José Luis Ramírez Ramírez, Mar 31 2013
Define an infinite triangle by T(n,0)=1 and the other cells by T(n,k) = Sum_{c=0..k-1} T(n,c) + Sum_{r=k..n-1} T(r,k), the sum of the cells to the left and above a cell. The column k=1 contains A000079, the column k=2 essentially A001792. Then T(n,n)=a(n) on the diagonal. - J. M. Bergot, May 22 2013

Examples

			Write [0...n]y for [x0,...,xn]y and [0...s,0...t]g for [x0,...,xs;y0,...,yt]g.
For n = 1 one finds 1 term, [01]y = -[01;1]g/[0;01]g.
For n = 2 one finds 3 terms, [012]y = -[012;2]g/[0;02]g + ([01;12]g[12;2]g)/([0;02]g[1;12]g) - ([0;012]g[01;1]g[12;2]g)/([0;02]g[0;01]g[1;12]g).

Links

Crossrefs

Cf. A172003, which is a generalization to bivariate implicit functions.
Cf. A003262, which is the analogous sequence for implicit derivatives, and A172004 for its generalization to bivariate implicit functions.
Cf. A082298, A000108, A151374.

Programs

  • Magma
    m:=20; R:=PowerSeriesRing(Rationals(), m); Coefficients(R!( (5-Sqrt((1-9*x)/(1-x)))/4 )); // G. C. Greubel, Feb 07 2019
  • Magma
    a:=[1,3]; for n in [3..21] do Append(~a,(2*(-7+5*n)*a[n-1] + 9*(2-n)*a[n-2]) div n); end for ; [1] cat a; // Marius A. Burtea, Jan 20 2020
  • Mathematica
    CoefficientList[Series[(5-Sqrt[(1-9*x)/(1-x)])/4, {x, 0, 20}], x] (* Vaclav Kotesovec, Oct 20 2012 *)
  • Maxima
    a(n):=if n=0 then 1 else sum(binomial(n,k)*2^(n-k-1)*binomial(2*n-2*k-2,n-k-1),k,0,n)/n; /* Vladimir Kruchinin, Mar 13 2016 */
  • PARI
    a(n) = if(n<2, 1, (2*(-7+5*n)*a(n-1) + 9*(2-n)*a(n-2))/n);
vector(25, n, a(n-1)) \\ Altug Alkan, Oct 06 2015
  • PARI
    my(x='x+O('x^20)); Vec((5-sqrt((1-9*x)/(1-x)))/4) \\ G. C. Greubel, Feb 07 2019
  • Python
    L = [1, 1]
for n in range(2,22):
    L.append( ((-14 + 10*n)*L[-1] + (18-9*n)*L[-2])//n )
print(L)
# Georg Muntingh, Jul 19 2012
  • Sage
    ((5-sqrt((1-9*x)/(1-x)))/4).series(x, 20).coefficients(x, sparse=False) # G. C. Greubel, Feb 07 2019

Formula

Let E = N x N \ {(0,0), (0,1)} be a set of pairs of natural numbers. The number of terms a(n) is the coefficient of x^n*y^{n-1} of the generating function 1 - log(1 - Sum_{(s,t) in E} x^s*y^{s+t-1}) = 1 + Sum_{q >= 1} (Sum_{(s,t) in E} x^s*y^{s+t-1})^q / q.
From Georg Muntingh, Jul 19 2012: (Start)
a(n) = 2F1(1/2,1-n;2;-8), where 2F1 is the Gauss hypergeometric series.
G.f.: (5 - sqrt( (1-9*x)/(1-x) ))/4.
Quadratic recurrence relation: a(n) = 1 + 2*Sum_{m=1..n-1} a(m)*a(n-m).
(End)
a(n) ~ 3^(2*n+1)/(16*sqrt(2*Pi)*n^(3/2)). - Vaclav Kotesovec, Oct 20 2012
a(n) = Sum_{k=0..n} (binomial(n,k)*2^(n-k-1)*binomial(2*n-2*k-2,n-k-1))/n, n>0, a(0)=1. - Vladimir Kruchinin, Mar 13 2016
From Peter Bala, Jan 19 2020: (Start)
a(n+1) = Sum_{k = 0..n} 2^k*C(n,k)*Catalan(k).
a(n+1) = (2/Pi) * Integral_{x = -1..1} (1 + 8*x^2)^n*sqrt(1 - x^2) dx.
O.g.f.: 1 + x/(1 - x)*c(2*x/(1-x)), where c(x) is the o.g.f. for A000108. (End)
Conjecture: a(n) = t_n for n > 0 with a(0) = 1 where we start with vector v of fixed length m with elements v_i = 1, then set t = v and for i=1..m-1, for j=i+1..m apply [v_i, v_j] := [v_i + 2*v_j, 2*v_i + v_j] (here square brackets mean that instead of sequentially assigning v_i and then v_j, we reserve their values (for example, as A = v_i, B = v_j) and then assign them in any order) and t_{i+1} := v_{i+1} (after ending each cycle for j). It also looks like that if we change 2*v_i to z*v_i it gives us a(n+1) = Sum_{k=0..n} A090981(n, k)*2^(n-k) for n >= 0. - Mikhail Kurkov, Aug 14 2024

Extensions

Edited by Georg Muntingh, Jan 22 2010

A125190 Number of ascents in all Schroeder paths of length 2n.

Original entry on oeis.org

0, 1, 6, 32, 170, 912, 4942, 27008, 148626, 822560, 4573910, 25534368, 143027898, 803467056, 4524812190, 25537728000, 144411206178, 818017823808, 4640757865126, 26364054632480, 149959897539018, 853941394691792, 4867745532495086, 27773897706129792
Offset: 0

Views

Author

Emeric Deutsch, Dec 20 2006

Keywords

Comments

A Schroeder path of length 2n is a lattice path in the first quadrant, from the origin to the point (2n, 0) and consisting of steps U = (1, 1), D = (1, -1) and H = (2, 0); an ascent in a Schroeder path is a maximal strings of U steps.
a(n) is the number of points at L1 distance n - 2 from any point in Z^n, for n >= 2. - Shel Kaphan, Mar 24 2023

Examples

			a(2) = 6 because the Schroeder paths of length 4 are HH, H(U)D, (U)DH, (U)D(U)D, (U)HD and (UU)DD, having a total of 6 ascents (shown between parentheses).

Links

Crossrefs

Cf. A006319, A090981, A008288, A026002, A190666.
-2-diagonal of A266213 for n>=1.

Programs

  • Maple
    R:=(1-z-sqrt(1-6*z+z^2))/2/z: G:=z*R*(1+z*R)/sqrt(1-6*z+z^2): Gser:=series(G,z=0,30): seq(coeff(Gser,z,n),n=0..25);
# second Maple program:
a:= proc(n) option remember;
      `if`(n<3, [0,1,6][n+1], ((204*n^2-411*n+198)*a(n-1)
       +(-34*n^2+68*n+96)*a(n-2) +(3*n-12)*a(n-3))/(2*n*(17*n-26)))
    end:
seq(a(n), n=0..30);  # Alois P. Heinz, Oct 20 2012
  • Mathematica
    CoefficientList[Series[x*(1-x-Sqrt[1-6*x+x^2])/(2*x)*(1+x*(1-x-Sqrt[1-6*x+x^2])/(2*x))/Sqrt[1-6*x+x^2], {x, 0, 20}], x] (* Vaclav Kotesovec, Oct 19 2012 *)
a[n_] := Sum[ Binomial[n+1, n-i-1]*Binomial[n+i, n], {i, 0, n-1}]; (* or *) a[n_] := Hypergeometric2F1[1-n, 1+n, 3, -1]*n*(n+1)/2; Table[a[n], {n, 0, 23}] (* Jean-François Alcover, Feb 05 2013, after Vladimir Kruchinin *)
  • Sage
    A125190 = lambda n : (n^2+n)*hypergeometric([1-n, n+1], [3], -1)/2
[round(A125190(n).n(100)) for n in (0..23)] # Peter Luschny, Sep 17 2014

Formula

a(n) = Sum_{k=0..n} k * A090981(n, k).
G.f.: z*R*(1 + z*R)/sqrt(1 - 6*z + z^2), where R = 1 + z*R + z*R^2, i.e., R = (1 - z -sqrt(1 - 6*z + z^2))/(2*z).
D-finite Recurrence: 2*n*(17*n - 26)*a(n) = 3*(68*n^2 - 137*n + 66)*a(n-1) - 2*(17*n^2 - 34*n - 48)*a(n-2) + 3*(n - 4)*a(n-3). - Vaclav Kotesovec, Oct 19 2012
a(n) ~ 2^(-3/4)*(3 + 2*sqrt(2))^n/sqrt(Pi*n). - Vaclav Kotesovec, Oct 19 2012
a(n) = Sum_{i=0..n-1} binomial(n+1, n-i-1) * binomial(n+i, n). - Vladimir Kruchinin, Feb 05 2013
a(n) = (n*(n+1)/2)*hypergeometric([1-n, n+1], [3], -1). - Peter Luschny, Sep 17 2014
a(n) = A026002(n) - A190666(n-2) for n >= 2. - Shel Kaphan, Mar 24 2023
a(n) = ((n+1)/2) * A006319(n-1). - Vladimir Kruchinin, Apr 27 2024
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