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A298647 Number of Dyck paths of semilength n having maximal degree of asymmetry, namely n-1 for n>2 and 0 otherwise.

%I A298647 #27 Feb 12 2021 18:47:00
%S A298647 1,1,2,2,2,8,16,52,134,432,1248,4104,12648,42464,136512,466568,
%T A298647 1545566,5361632,18165184,63804952,219997832,780895392,2730730176,
%U A298647 9780049008,34598622616,124873507904,446068180608,1620786592416,5837657948832,21336295622016,77395590570240
%N A298647 Number of Dyck paths of semilength n having maximal degree of asymmetry, namely n-1 for n>2 and 0 otherwise.
%C A298647 The degree of asymmetry of a Dyck path is defined in the following manner: we label the steps of a Dyck path of length 2n, from left to right, by 1,2,..., n-1, n, n, n-1, ..., 2,1. The degree of asymmetry is defined to be the number of pairs of identically labeled steps that are not at the same level. Example: the Dyck path UDUUDD has degree of asymmetry 2. Indeed, the labels are 123321 and the steps labeled 2 are at different levels and those labeled 3 are also at different levels.
%H A298647 Alois P. Heinz, <a href="/A298647/b298647.txt">Table of n, a(n) for n = 0..1676</a>
%F A298647 a(n) = A298645(n,n-1) for n > 2.
%F A298647 a(n) ~ 2^(2*n + 3) / (Pi * n^3). - _Vaclav Kotesovec_, Mar 06 2018
%e A298647 a(1) = 1, counting UD; a(2) = 2 since both UDUD and UUDD have maximal degree of asymmetry 0; a(5) = 8 counting UDUUUUDDDD, UDUDUUUDDD, UDUUDDUUDD, UDUDUUDUDD, and their reflections in a vertical axis.
%p A298647 b:= proc(x, y, v) option remember; expand(
%p A298647       `if`(min(y, v, x-max(y, v))<0, 0, `if`(x=0, 1, (l-> add(add(
%p A298647       `if`(y=v+(j-i)/2, 1, z)*b(x-1, y+i, v+j), i=l), j=l))([-1, 1]))))
%p A298647     end:
%p A298647 a:= n-> lcoeff(add(b(n, j$2), j=0..n)):
%p A298647 seq(a(n), n=0..40);
%p A298647 # second Maple program:
%p A298647 a:= proc(n) option remember; `if`(n<8, [1$2, 2$3, 8, 16, 52][n+1],
%p A298647       (256*(n-4)*(n-5)*(n-8)*(147*n^5-1708*n^4+7165*n^3-12896*n^2+8882*n
%p A298647       -1362)*a(n-6)-64*(294*n^7-6139*n^6+52088*n^5-227713*n^4+534530*n^3
%p A298647       -630478*n^2+295718*n-24240)*a(n-5)-16*(3675*n^8-96943*n^7+1072857
%p A298647       *n^6-6448749*n^5+22718880*n^4-46984656*n^3+53484228*n^2-28042052*n
%p A298647       +3963360)*a(n-4)-24*(1470*n^7-30345*n^6+256611*n^5-1138707*n^4
%p A298647       +2807363*n^3-3706740*n^2+2239444*n-369120)*a(n-3)+4*(n-2)*(1470
%p A298647       *n^7-27958*n^6+211755*n^5-810763*n^4+1642737*n^3-1686751*n^2+790686
%p A298647       *n-150480)*a(n-2)+2*(n-1)*(882*n^6-14357*n^5+89078*n^4-262023*n^3
%p A298647       +370096*n^2-233344*n+64320)*a(n-1))/((147*n^5-2443*n^4+15467*n^3
%p A298647       -46109*n^2+63736*n-32160)*(n-1)*n*(n+1)))
%p A298647     end:
%p A298647 seq(a(n), n=0..40);
%t A298647 b[x_, y_, v_] := b[x, y, v] = Expand[
%t A298647   If[Min[y, v, x - Max[y, v]]<0, 0, If[x==0, 1, Function[l, Sum[Sum[
%t A298647   If[y == v+(j-i)/2, 1, z] b[x-1, y+i, v+j], {i, l}], {j, l}]][{-1, 1}]]]];
%t A298647 a[n_] := With[{p = Sum[b[n, j, j], {j, 0, n}]}, Coefficient[p, z, Exponent[p, z]]];
%t A298647 a /@ Range[0, 40] (* _Jean-François Alcover_, Dec 30 2020, after _Alois P. Heinz_ *)
%Y A298647 Cf. A298645, A298646.
%Y A298647 Column k=1 of A341445 (for n>2).
%K A298647 nonn
%O A298647 0,3
%A A298647 _Emeric Deutsch_ and _Alois P. Heinz_, Feb 21 2018