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A325927
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Number of Motzkin meanders of length n with an odd number of humps and an odd number of peaks.
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4
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0, 0, 1, 4, 13, 38, 105, 280, 737, 1942, 5183, 14100, 39151, 110642, 316751, 914248, 2650655, 7701562, 22400559, 65203428, 189970159, 554165922, 1619018259, 4737859512, 13887657307, 40769959314, 119849273449, 352716050428, 1039027117929
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OFFSET
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0,4
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COMMENTS
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A Motzkin meander is a lattice path with steps from the set {D=-1, H=0, U=1} that starts at (0,0), and never goes below the x-axis.
A peak is an occurrence of the pattern UD.
A hump is an occurrence of the pattern UHH...HD (the number of Hs in the pattern is not fixed, and can be 0).
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LINKS
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FORMULA
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G.f.: ( sqrt((1+t)/(1-3*t)) - sqrt((1+t+2*t^2)/((1-2*t)*(1-t))) - sqrt((1+t^2)/(1-4*t+5*t^2)) + sqrt((1-t^2+2*t^3)/((1-2*t)*(1-t^2-2*t))) ) / (8*t).
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EXAMPLE
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For n=3, the a(3)=4 paths are UDH, UDU, UUD, HUD (1 hump, 1 peak).
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PROG
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(PARI) seq(n)={my(t='x + O('x*'x^n)); Vec(( sqrt((1+t)/(1-3*t)) - sqrt((1+t+2*t^2)/((1-2*t)*(1-t))) - sqrt((1+t^2)/(1-4*t+5*t^2)) + sqrt((1-t^2+2*t^3)/((1-2*t)*(1-t^2-2*t))) ) / (8*t), -n)} \\ Andrew Howroyd, Aug 12 2019
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CROSSREFS
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Motzkin meanders and excursions with parity restrictions on the number of humps and peaks:
A325921: Meanders, #humps=EVEN, #peaks=EVEN.
A325922: Excursions, #humps=EVEN, #peaks=EVEN.
A325923: Meanders, #humps=ODD, #peaks=EVEN.
A325924: Excursions, #humps=ODD, #peaks=EVEN.
A325925: Meanders, #humps=EVEN, #peaks=ODD.
A325926: Excursions, #humps=EVEN, #peaks=ODD.
A325927 (this sequence): Meanders, #humps=ODD, #peaks=ODD.
A325928: Excursions, #humps=ODD, #peaks=ODD.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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