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 A191792 Number of length n left factors of Dyck paths having no UDUD's; here U=(1,1) and D=(1,-1). 1
 1, 1, 2, 3, 5, 8, 15, 25, 46, 79, 147, 256, 477, 841, 1570, 2791, 5217, 9336, 17467, 31421, 58830, 106279, 199103, 360960, 676545, 1230185, 2306642, 4204931, 7887045, 14409480, 27035135, 49487641, 92872062, 170289575, 319647235, 586983680, 1102027213, 2026422689, 3805138290 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS a(n) = A191791(n,0). LINKS FORMULA G.f.: g(z)=C/(1-z*C), where C=C(z) is given by z^2*(1+z^2)*C^2-(1+z^2+z^4)*C+1+z^2=0. EXAMPLE a(4)=5 because we have UDUU, UUDD, UUDU, UUUD, and UUUU, where U=(1,1) and D=(1,-1) (the path UDUD does not qualify). MAPLE eq := z^2*(1+z^2)*C^2-(1+z^2+z^4)*C+1+z^2 = 0: C := RootOf(eq, C): g := C/(1-z*C): gser := series(g, z = 0, 42): seq(coeff(gser, z, n), n = 0 .. 38); CROSSREFS Cf. A191791. Sequence in context: A058519 A181065 A282239 * A151518 A082095 A177486 Adjacent sequences:  A191789 A191790 A191791 * A191793 A191794 A191795 KEYWORD nonn AUTHOR Emeric Deutsch, Jun 18 2011 STATUS approved

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Last modified July 8 01:42 EDT 2020. Contains 335502 sequences. (Running on oeis4.)