A151420 - OEIS

%I #7 Jan 01 2024 02:43:44

%S 1,1,1,3,9,20,60,196,589,1907,6516,21860,75589,267566,950279,3423629,

%T 12499661,45919855,170274916,636617648,2394733039,9067049073,

%U 34535991913,132213431523,508684283599,1966124324293,7630970742790,29734868078582,116292627615997,456375717434690,1796789652244346

%N Number of walks within N^2 (the first quadrant of Z^2) starting at (0,0), ending on the vertical axis and consisting of n steps taken from {(-1, -1), (-1, 0), (-1, 1), (0, 1), (1, -1)}.

%H M. Bousquet-Mélou and M. Mishna, 2008. Walks with small steps in the quarter plane, <a href="http://arxiv.org/abs/0810.4387">ArXiv 0810.4387</a>.

%t aux[i_Integer, j_Integer, n_Integer] := Which[Min[i, j, n] < 0 || Max[i, j] > n, 0, n == 0, KroneckerDelta[i, j, n], True, aux[i, j, n] = aux[-1 + i, 1 + j, -1 + n] + aux[i, -1 + j, -1 + n] + aux[1 + i, -1 + j, -1 + n] + aux[1 + i, j, -1 + n] + aux[1 + i, 1 + j, -1 + n]]; Table[Sum[aux[0, k, n], {k, 0, n}], {n, 0, 25}]

%K nonn,walk

%O 0,4

%A _Manuel Kauers_, Nov 18 2008