A361190 Number of 4n-step lattice paths starting and ending at (0,0) that do not go above the diagonal x=y or below the x-axis using steps in {(1,1), (1,-1), (-1,0)}.

1, 1, 9, 153, 3579, 101630, 3288871, 116951012, 4465824585, 180310624841, 7614208325878, 333613510494834, 15075162152856423, 699290488810583617, 33176816563410874752, 1605135467691243954419, 79003021319962788395355, 3947913343912428255683930

Offset: 0

Author

Alois P. Heinz, Jul 31 2023

Keywords

• nonn
• walk

Comments

Is this the same sequence as A217823?

Examples

			a(0) = 1: (00), 0 steps are made.
a(1) = 1: (00)(11)(20)(10)(00).
a(2) = 9:
  (00)(11)(20)(10)(00)(11)(20)(10)(00),
  (00)(11)(20)(10)(21)(30)(20)(10)(00),
  (00)(11)(20)(10)(21)(11)(20)(10)(00),
  (00)(11)(20)(31)(40)(30)(20)(10)(00),
  (00)(11)(20)(31)(21)(30)(20)(10)(00),
  (00)(11)(20)(31)(21)(11)(20)(10)(00),
  (00)(11)(22)(31)(40)(30)(20)(10)(00),
  (00)(11)(22)(31)(21)(30)(20)(10)(00),
  (00)(11)(22)(31)(21)(11)(20)(10)(00).
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..400
• Wikipedia, Lattice path

Crossrefs

Cf. A001006, A005789, A026945, A151332 (the same without condition on the diagonal), A217823, A359647.

Programs

Maple

b:= proc(n, x, y) option remember; `if`(x+2*y>n, 0,
     `if`(n=0, 1, `if`(y>0, b(n-1, x+1, y-1), 0)+
     `if`(y b(4*n, 0$2):
seq(a(n), n=0..17);

Formula

a(n) = A217823(n) for n<=6.