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A329673
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Number of meanders of length n with Motzkin-steps avoiding the consecutive steps HH.
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2
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1, 2, 4, 10, 24, 60, 152, 388, 1000, 2592, 6752, 17664, 46368, 122080, 322240, 852464, 2259552, 5999552, 15954560, 42486592, 113282048, 302386304, 807999744, 2161077120, 5785032448, 15498450944, 41551965184, 111478804480, 299274439680, 803905119232, 2160632498176, 5810087371520
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OFFSET
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0,2
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COMMENTS
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The Motzkin step set is U=(1,1), H=(1,0) and D=(1,-1). A meander is a path starting at (0,0) and never crossing the x-axis, i.e., staying at nonnegative altitude.
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LINKS
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FORMULA
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G.f.: -(1-2*t-2*t^2-sqrt(1-4*t^2-8*t^3-4*t^4))/(2*t*(1-2*t-2*t^2)).
D-finite with recurrence (n+1)*a(n) -2*a(n-1) -4*n*a(n-2) +8*(-n+2)*a(n-3) +4*(-n+3)*a(n-4)=0. - R. J. Mathar, Jan 25 2023
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EXAMPLE
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a(2)=4 since we have 4 meanders of length 2 avoiding HH, namely UU, UH, UD and HU.
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CROSSREFS
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Cf. A104545 which counts excursions avoiding consecutive HH steps. Cf. A329672 and A329674 which count meanders avoiding consecutive UU and DD respectively.
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KEYWORD
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nonn,walk
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AUTHOR
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STATUS
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approved
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