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A247333 Number of Dyck paths of semilength n avoiding the consecutive step pattern UDUDU, where U=(1,1) and D=(1,-1). 3
1, 1, 2, 4, 11, 31, 92, 283, 893, 2875, 9407, 31189, 104555, 353794, 1206821, 4145350, 14326184, 49778473, 173794610, 609392578, 2145057797, 7577098816, 26850456704, 95425761829, 340047930692, 1214738997142, 4349231444405, 15604726428805, 56098211626478 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..1000

FORMULA

Recursion: see Maple program.

a(n) ~ sqrt(42-6*sqrt(21)) * ((3+sqrt(21))/2)^n / (4 * sqrt(Pi) * n^(3/2)). - Vaclav Kotesovec, Sep 16 2014

G.f.: (1+x+x^2-sqrt(-3*x^4-6*x^3-5*x^2-2*x+1))/(2*x^2+2*x). - Vladimir Kruchinin, Mar 03 2016

a(n) = Sum_{k=1..n} binomial(k,n-k)*M(k+1), a(0)=1, where M(k) is Motzkin numbers (A001006). - Vladimir Kruchinin, Mar 03 2016

A(x)=B(x)+1, where B(x) satisfies of B(x)=(x+x^2)*(1+B(x)+B(x)^2). - Vladimir Kruchinin, Mar 03 2016

MAPLE

a:= proc(n) option remember; `if`(n<5, [1$2, 2, 4, 11][n+1],

      ((n-2)*a(n-1) +(7*n-11)*a(n-2) +(11*n-31)*a(n-3)

      +(9*n-36)*a(n-4) +(3*n-15)*a(n-5)) / (n+1))

    end:

seq(a(n), n=0..40);

MATHEMATICA

CoefficientList[Series[(1 + x + x^2 - Sqrt[-3 x^4 - 6 x^3 - 5 x^2 - 2 x + 1])/(2 x^2 + 2 x), {x, 0, 28}], x] (* or *)

{1}~Join~Table[Sum[(Binomial[k, n - k] Sum[Binomial[j, -k + 2 j - 1] Binomial[k, j], {j, 0, k}])/k, {k, 1, n}], {n, 1, 28}] (* Michael De Vlieger, Mar 03 2016, the latter after Maxima by Vladimir Kruchinin *)

PROG

(Maxima)

a(n):=if n=0 then 1 else sum((binomial(k, n-k)*sum(binomial(j, -k+2*j-1)*binomial(k, j), j, 0, k))/k, k, 1, n); /* Vladimir Kruchinin, Mar 03 2016 */

CROSSREFS

Cf. A001006.

Column k=0 of A246188.

Column k=21 of A243753.

Sequence in context: A148163 A274775 A039300 * A118974 A119020 A073191

Adjacent sequences:  A247330 A247331 A247332 * A247334 A247335 A247336

KEYWORD

nonn

AUTHOR

Alois P. Heinz, Sep 13 2014

STATUS

approved

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Last modified September 25 15:21 EDT 2021. Contains 347657 sequences. (Running on oeis4.)