A187260 Number of uh^jd's for some j>0, starting at level 0, where u=(1,1), h=(1,0), and d=(1,-1), in all peakless Motzkin paths of length n (can be easily expressed using RNA secondary structure terminology).
0, 0, 0, 1, 3, 6, 12, 25, 53, 115, 255, 575, 1315, 3043, 7111, 16756, 39766, 94961, 228003, 550081, 1332839, 3241930, 7913028, 19375635, 47579847, 117149125, 289142441, 715253644, 1773011502, 4403539181, 10956537307, 27307002454, 68164324150, 170404155586, 426584025250, 1069289177950
Offset: 0
Keywords
Examples
a(4)=3 because the 4 (=A004148(4)) peakless Motzkin paths of length 4, namely hhhh, h(uhd), (uhd)h, and (uhhd) contain 0+1+1+1 subwords of type uh^jd for some j>0, starting at level 0 (shown between parentheses).
Programs
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Maple
eq := g = 1+z*g+z^2*g*(g-1): g := RootOf(eq, g): F := z^3*g^2/(1-z): Fser := series(F, z = 0, 38): seq(coeff(Fser, z, n), n = 0 .. 35);
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Mathematica
CoefficientList[Series[(-1 + x - x^2 + Sqrt[(1 + (-3 + x)*x)*(1 + x + x^2)])^2 / (4*(1 - x)*x), {x, 0, 40}], x] (* Vaclav Kotesovec, May 29 2022 *)
Formula
G.f.: z^3*g^2/(1-z), where g=1+z*g+z^2*g*(g-1).
a(n) = Sum_{k>=0} k*A098071(n,k).
From Vaclav Kotesovec, May 29 2022: (Start)
G.f.: (-1 + x - x^2 + sqrt((1 + (-3 + x)*x) * (1 + x + x^2)))^2 / (4*(1-x)*x).
a(n) ~ 5^(1/4) * phi^(2*n-1) / (sqrt(Pi) * n^(3/2)), where phi = A001622 is the golden ratio. (End)
D-finite with recurrence (n+1)*a(n) +(-4*n+1)*a(n-1) +(5*n-8)*a(n-2) +(-5*n+18)*a(n-3) +(5*n-22)*a(n-4) +(-5*n+32)*a(n-5) +(4*n-31)*a(n-6) +(-n+9)*a(n-7)=0. - R. J. Mathar, Jul 22 2022
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