0,4

Essentially the same as A025246.

Number of lattice paths in the first quadrant from (0,0) to (n,0) using only steps H=(1,0), U=(1,1) and D=(2,-1). E.g. a(5)=7 because we have HHHHH, HHUD, HUDH, HUHD, UDHH, UHDH and UHHD. - Emeric Deutsch, Dec 25 2003

Also number of peakless Motzkin paths of length n with no double rises; in other words, Motzkin paths of length n with no UD's and no UU's, where U=(1,1) and D=(1,-1). E.g. a(5)=7 because we have HHHHH, HHUHD, HUHDH, HUHHD, UHDHH, UHHDH and UHHHD, where H=(1,0). - Emeric Deutsch, Jan 09 2004

Series reversion of g.f. A(x) is -A(-x) (if offset 1). - Michael Somos, Jul 13 2003

Hankel transform is A010892(n+1). [From Paul Barry, Sep 19 2008]

Number of FU_{k}-equivalence classes of Łukasiewicz paths. Łukasiewicz paths are P-equivalent iff the positions of pattern P are identical in these paths. This also works for U_{k}F-equivalence classes. - Sergey Kirgizov, Apr 08 2018

Reinhard Zumkeller, Table of n, a(n) for n = 0..1000

Andrei Asinowski, Cyril Banderier, and Valerie Roitner, Generating functions for lattice paths with several forbidden patterns, (2019).

Jean-Luc Baril, Sergey Kirgizov and Armen Petrossian, Enumeration of Łukasiewicz paths modulo some patterns, arXiv:1804.01293 [math.CO], 2018.

Paul Barry, Continued fractions and transformations of integer sequences, JIS 12 (2009) 09.7.6.

Paul Barry, Jacobsthal Decompositions of Pascal's Triangle, Ternary Trees, and Alternating Sign Matrices, Journal of Integer Sequences, 19, 2016, #16.3.5.

Paul Barry, Riordan Pseudo-Involutions, Continued Fractions and Somos 4 Sequences, arXiv:1807.05794 [math.CO], 2018.

Paul Barry, Generalized Catalan recurrences, Riordan arrays, elliptic curves, and orthogonal polynomials, arXiv:1910.00875 [math.CO], 2019.

Paul Barry, Elliptic Curves, Riordan arrays and Lattice Paths, arXiv:2507.16765 [math.CO], 2025. See p. 16.

Johann Cigler, Some nice Hankel determinants, arXiv preprint arXiv:1109.1449 [math.CO], 2011.

Shalosh B. Ekhad and Mingjia Yang, Proofs of Linear Recurrences of Coefficients of Certain Algebraic Formal Power Series Conjectured in the On-Line Encyclopedia Of Integer Sequences, (2017)

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 666

Feihu Liu, Ying Wang, Yingrui Zhang, and Zihao Zhang, Hankel Determinants for Convolution of Power Series: An Extension of Cigler's Results, arXiv:2503.17187 [math.CO], 2025. See p. 9.

K. Park and G.-S. Cheon, Lattice path counting with a bounded strip restriction

Helmut Prodinger, Motzkin paths of bounded height with two forbidden contiguous subwords of length two, arXiv:2310.12497 [math.CO], 2023.

G.f.: (1 - x - sqrt((1-x)^2 - 4*x^3)) / (2*x^3) = A(x). y = x * A(x) satisfies 0 = x - y + x*y + (x*y)^2. - Michael Somos, Jul 13 2003

a(n+1) = a(n) + a(0)*a(n-2) + a(1)*a(n-3) + ... + a(n-2)*a(0). - Michael Somos, Jul 13 2003

a(n) = A025246(n+3). - Michael Somos, Jan 20 2004

G.f.: (1/(1-x))*c(x^3/(1-x)^2), c(x) the g.f. of A000108. - From Paul Barry, Sep 19 2008

From Paul Barry, May 22 2009: (Start)

G.f.: 1/(1-x-x^3/(1-x-x^3/(1-x-x^3/(1-x-x^3/(1-... (continued fraction).

a(n) = Sum_{k=0..floor(n/3)} binomial(n-k, 2*k)*A000108(k). (End)

(n+3)*a(n) = (2*n+3)*a(n-1) - n*a(n-2) + 2*(2*n-3)*a(n-3). - R. J. Mathar, Nov 26 2012

0 = a(n)*(16*a(n+1) - 10*a(n+2) + 32*a(n+3) - 22*a(n+4)) + a(n+1)*(2*a(n+1) - 15*a(n+2) + 9*a(n+3) + 4*a(n+4)) + a(n+2)*(a(n+2) + 2*a(n+3) - 5*a(n+4)) + a(n+3)*(a(n+3) + a(n+4)) if n>=0. - Michael Somos, Jan 30 2014

a(n) ~ (8 + 12*r^2 + 5*r) * sqrt(r^2 - 4*r + 3) / (4 * sqrt(Pi) * n^(3/2) * r^n), where r = 0.432040800333095789... is the real root of the equation -1 + 2*r - r^2 + 4*r^3 = 0. - Vaclav Kotesovec, Jun 15 2022

a(n) = hypergeom([(1 - n)/3, (2 - n)/3, -n/3], [2, -n], 27). - Peter Luschny, Jun 15 2022

G.f. = 1 + x + x^2 + 2*x^3 + 4*x^4 + 7*x^5 + 13*x^6 + 26*x^7 + 52*x^8 + 104*x^9 + ...

a := n -> hypergeom([1/3 - n/3, 2/3 - n/3, -n/3], [2, -n], 27):

seq(simplify(a(n)), n = 0..32); # Peter Luschny, Jun 15 2022

a[0]=1; a[n_]:= a[n]= a[n-1] + Sum[a[k]*a[n-3-k], {k, 0, n-3}];

Table[a[n], {n, 0, 40}]

(PARI) {a(n) = polcoeff( (1 - x - sqrt((1-x)^2 - 4*x^3 + x^4 * O(x^n))) / 2, n+3)}; /* Michael Somos, Jul 13 2003 */

(Haskell)

a023431 n = a023431_list !! n

a023431_list = 1 : 1 : f [1, 1] where

f xs'@(x:_:xs) = y : f (y : xs') where

y = x + sum (zipWith (*) xs $ reverse $ xs')

-- Reinhard Zumkeller, Nov 13 2012

(Magma) [(&+[Binomial(n-k, 2*k)*Catalan(k): k in [0..Floor(n/3)]]): n in [0..40]]; // G. C. Greubel, Jun 15 2022

(SageMath) [sum(binomial(n-k, 2*k)*catalan_number(k) for k in (0..(n//3))) for n in (0..40)] # G. C. Greubel, Jun 15 2022

Cf. A000108, A001006, A004148, A006318, A025246.

Cf. A014137, A090344, A144700.

Sequence in context: A262267 A068031 A293314 * A025246 A256942 A112740

Adjacent sequences: A023428 A023429 A023430 * A023432 A023433 A023434

nonn,easy

approved