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A151367
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Number of walks within N^2 (the first quadrant of Z^2) starting and ending at (0,0) and consisting of n steps taken from {(-1, -1), (-1, 0), (-1, 1), (0, -1), (0, 1), (1, 1)}.
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1
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1, 0, 2, 2, 13, 27, 140, 392, 1882, 6289, 28906, 107949, 486438, 1948638, 8730438, 36611160, 164259758, 710530289, 3203433595, 14163150429, 64260242637, 288694503092, 1318679597635, 5996837692998, 27572301084897, 126595556379751, 585652882733959, 2709967750078764, 12607711205847168
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OFFSET
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0,3
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LINKS
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MAPLE
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Steps:= [[-1, -1], [-1, 0], [-1, 1], [0, -1], [0, 1], [1, 1]]:
f:= proc(n, p) option remember; local t, s;
if max(p) > n then return 0 fi;
add(procname(n-1, s), s = select(t -> min(t)>=0, map(`+`, Steps, p)))
end proc:
f(0, [0, 0]):= 1:
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MATHEMATICA
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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[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[aux[0, 0, n], {n, 0, 25}]
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CROSSREFS
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KEYWORD
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nonn,walk,changed
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AUTHOR
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STATUS
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approved
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