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%I A151409 #7 Jun 20 2022 20:47:07
%S A151409 1,1,2,5,14,39,116,364,1154,3712,12245,40940,137779,468927,1612511,
%T A151409 5576133,19394557,67912071,238967041,844127643,2994719855,10665904225,
%U A151409 38103524211,136531060288,490663266986,1767779916316,6383507664304,23102978335829,83786075774791,304420859569058,1108009834275929
%N A151409 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, 0), (0, 1), (1, -1), (1, 1)}.
%H A151409 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 A151409 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[-1 + i, 1 + j, -1 + n] + aux[i, -1 + j, -1 + n] + aux[1 + i, j, -1 + n]]; Table[Sum[aux[0, k, n], {k, 0, n}], {n, 0, 25}]
%K A151409 nonn,walk
%O A151409 0,3
%A A151409 _Manuel Kauers_, Nov 18 2008

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