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 A086581 Number of Dyck paths of semilength n with no DDUU. 10
 1, 1, 2, 5, 13, 35, 97, 275, 794, 2327, 6905, 20705, 62642, 190987, 586219, 1810011, 5617914, 17518463, 54857506, 172431935, 543861219, 1720737981, 5459867166, 17369553427, 55391735455, 177040109419, 567019562429, 1819536774089 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS See A025242 for a bijection between paths avoiding UUDD versus DDUU. Number of lattice paths, never going below the x-axis, from (0,0) to (n,0) consisting of up steps U(k) = (k,1) for every positive integer k, down steps D = (1,-1) and horizontal steps H. - José Luis Ramírez Ramírez, Apr 19 2015 Given a sequence variant with 0 inserted between the two 1's, the INVERT transform of the modified sequence is this sequence. - Gary W. Adamson, Jun 28 2015 LINKS Robert Israel, Table of n, a(n) for n = 0..1709 P Barry, Continued fractions and transformations of integer sequences, JIS 12 (2009) 09.7.6 T. Mansour, Restricted 1-3-2 permutations and generalized patterns, arXiv:math/0110039 [math.CO], 2001. T. Mansour, Restricted 1-3-2 permutations and generalized patterns, Annals of Combin., 6 (2002), 65-76. (Example 2.10.) L. Pudwell, Pattern avoidance in trees (slides from a talk, mentions many sequences), 2012. - From N. J. A. Sloane, Jan 03 2013 A. Sapounakis, I. Tasoulas and P. Tsikouras, On the Dominance Partial Ordering of Dyck Paths, Journal of Integer Sequences, Vol. 9 (2006), Article 06.2.5. A. Sapounakis, I. Tasoulas and P. Tsikouras, Counting strings in Dyck paths, Discrete Math., 307 (2007), 2909-2924. Murray Tannock, Equivalence classes of mesh patterns with a dominating pattern, MSc Thesis, Reykjavik Univ., May 2016. FORMULA G.f. A(x) satisfies the equation 0 = 1 - x - (1 - x)^2 * A(x) + (x * A(x))^2. a(n) = A025242(n+1) = A082582(n+1). G.f.: (1 - 2*x + x^2 - sqrt(1 - 4*x + 2*x^2 + x^4))  /(2 * x^2). a(n+2) - 2*a(n+1) + a(n) = a(0)*a(n) + a(1)*a(n-1) + ... + a(n)*a(0). G.f.: (1/(1-x))*c(x^2/(1-x)^3), c(x) the g.f. of A000108; a(n)=sum{k=0..floor(n/2), C(n+k,3k)*A000108(k)}. - Paul Barry, May 31 2006 Conjecture: (n+2)*a(n) +(n+3)*a(n-1) +2*(-9*n+4)*a(n-2) +10*(n-2)*a(n-3) +(n-4)*a(n-4) +5*(n-5)*a(n-5)=0. - R. J. Mathar, Nov 26 2012 G.f. satisfies (10*x^3-28*x^2+4*x+2)*A(x) + (5*x^6+x^5+10*x^4-18*x^3+x^2+x)*A'(x) = 5*x^4+x^3-15*x^2+7*x+2. This confirms R. J. Mathar's recurrence equation. - Robert Israel, Jun 29 2015 G.f.: 1 - G(0), where G(k)= 1 - 1/(1 - x/(1 - x/(1 - x/(1 - x/(x - 1/G(k+1) ))))); (continued fraction). - Sergei N. Gladkovskii, Jul 12 2013 G.f.: 1/G(0) where G(k) = 1 - q/(1 - q - q^2 / G(k+1) ); (continued fraction). - Joerg Arndt, Feb 27 2014 a(n) = 1+sum(k=0..n, sum(i=0..k, C(n-1,k)*C(2i+2,i)*C(i+2,k-2i-1)/(i+1))). - Thomas Baruchel, Jan 19 2015 a(n) = sum(k=0..n, C(2k,k) C(n+k,3k) / (k+1). - Thomas Baruchel, Jan 19 2015 sum(k=0..n, a(k+1) * A108626(n-k)) = sum(k=0..n, sum(i=0..k, binomial(n-k+1,i-1)*binomial(n-k+1,i)*binomial(n-i+1,k-i))). - Thomas Baruchel, Jan 19 2015 EXAMPLE a(4) = 13 because of the 14 Dyck 4-paths only UUDDUUDD contains DDUU. MAPLE F:= gfun:-rectoproc({(n+2)*a(n) +(n+3)*a(n-1) +2*(-9*n+4)*a(n-2) +10*(n-2)*a(n-3) +(n-4)*a(n-4) +5*(n-5)*a(n-5)=0, seq(a(n)=[1, 1, 2, 5, 13][n+1], n=0..4)}, a(n), remember): map(F, [\$0..30]); # Robert Israel, Jun 29 2015 MATHEMATICA CoefficientList[ Series[(1 - 2 x + x^2 - Sqrt[1 - 4 x + 2 x^2 + x^4])/(2 x^2), {x, 0, 27}], x] (* Robert G. Wilson v, Mar 25 2011 *) PROG (PARI) {a(n) = polcoeff((1 - 2*x + x^2 - sqrt(1 - 4*x + 2*x^2 + x^4 + x^3 * O(x^n))) / 2, n+2)} (PARI) a(n)=1+sum(k=0, n, sum(i=0, k, binomial(n-1, k)*binomial(2*i+2, i)*binomial(i+2, k-2*i-1)/(i+1))) \\ Thomas Baruchel, Jan 19 2015 CROSSREFS Cf. A025242, A082582. Column k=0 of A114492. Sequence in context: A007689 A085281 A082582 * A059027 A025198 A221205 Adjacent sequences:  A086578 A086579 A086580 * A086582 A086583 A086584 KEYWORD nonn AUTHOR Michael Somos, Jul 22 2003 EXTENSIONS Name corrected by David Scambler, Mar 28 2011 STATUS approved

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Last modified March 25 17:45 EDT 2019. Contains 321477 sequences. (Running on oeis4.)