%I #28 Jul 01 2018 08:38:16

%S 1,4,22,146,1013,7269,53156,394154,2951950,22279439,169175927,

%T 1290970376,9891573310,76050920691,586426828071,4533349152056,

%U 35122039919110,272634162463779,2119948044144136,16509519223752380,128747868290672353,1005273235488567875

%N Number of positive meanders (walks starting at the origin and ending at any altitude > 0 that never touch or go below the x-axis in between) with n steps from {-4,-3,-2,-1,1,2,3,4}.

%C By convention, the empty walk (corresponding to n=0) is considered to be a positive meander.

%H Andrew Howroyd, <a href="/A278396/b278396.txt">Table of n, a(n) for n = 0..200</a>

%H C. Banderier, C. Krattenthaler, A. Krinik, D. Kruchinin, V. Kruchinin, D. Nguyen, and M. Wallner, <a href="https://arxiv.org/abs/1609.06473">Explicit formulas for enumeration of lattice paths: basketball and the kernel method</a>, arXiv:1609.06473 [math.CO], 2016.

%t frac[ex_] := Select[ex, Exponent[#, x] < 0&];

%t seq[n_] := Module[{v, m, p}, v = Table[0, n]; m = Sum[x^i, {i, -4, 4}] - 1; p = 1/x; v[[1]] = 1; For[i = 2, i <= n, i++, p = p*m // Expand; p = p - frac[p]; v[[i]] = p /. x -> 1]; v];

%t seq[22] (* _Jean-François Alcover_, Jul 01 2018, after _Andrew Howroyd_ *)

%o (PARI) seq(n)={my(v=vector(n), m=sum(i=-4, 4, x^i)-1, p=1/x); v[1]=1; for(i=2, n, p*=m; p-=frac(p); v[i]=subst(p,x,1)); v} \\ _Andrew Howroyd_, Jun 27 2018

%Y Cf. A276852, A278391, A278392, A278393, A278394, A278395, A278398.

%K nonn,walk

%O 0,2

%A _David Nguyen_, Nov 20 2016