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A288680 Number of Dyck paths of semilength n such that no positive level has fewer than four peaks.

%I A288680 #15 Apr 24 2021 08:44:45
%S A288680 1,0,0,0,1,1,1,1,1,6,57,247,718,1795,4210,9969,27596,98507,402924,
%T A288680 1626525,6142611,21729644,73308577,241270869,793679894,2666563900,
%U A288680 9263663359,33259282181,122178034000,453573262015,1685632454779,6240174176549,22987207140830
%N A288680 Number of Dyck paths of semilength n such that no positive level has fewer than four peaks.
%H A288680 Alois P. Heinz, <a href="/A288680/b288680.txt">Table of n, a(n) for n = 0..300</a>
%H A288680 Wikipedia, <a href="https://en.wikipedia.org/wiki/Lattice_path#Counting_lattice_paths">Counting lattice paths</a>
%t A288680 b[n_, k_, j_]:=b[n, k, j]=If[j==n, 1, Sum[Sum[Binomial[i, m] Binomial[j - 1, i - 1 - m], {m, Max[k, i - j], i - 1}] b[n - j, k, i], {i, n - j}]];  a[n_]:=If[n==0, 1, Sum[b[n, 4, j], {j, 4, n}]];Table[a[n], {n, 0, 35}] (* _Indranil Ghosh_, Aug 09 2017 *)
%o A288680 (Python)
%o A288680 from sympy.core.cache import cacheit
%o A288680 from sympy import binomial
%o A288680 @cacheit
%o A288680 def b(n, k, j): return 1 if j==n else sum([sum([binomial(i, m)*binomial(j - 1, i - 1 - m) for m in range(max(k, i - j), i)])*b(n - j, k, i) for i in range(1, n - j + 1)])
%o A288680 def a(n): return 1 if n==0 else sum([b(n, 4, j) for j in range(4, n + 1)])
%o A288680 print([a(n) for n in range(36)]) # _Indranil Ghosh_, Aug 09 2017
%Y A288680 Column k=4 of A288386.
%Y A288680 Cf. A000108.
%K A288680 nonn
%O A288680 0,10
%A A288680 _Alois P. Heinz_, Jun 13 2017