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b[x_, y_] := b[x, y] = If[y > x, 0, If[x == 0, 1, If[y > 0, b[x, y - 1] + b[x - 1, y - 1], 0] + b[x - 1, y + 1]]];
a[n_] := b[n, n];
a /@ Range[0, 30] (* Jean-François Alcover, Dec 18 2020, after Alois P. Heinz *)
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Alois P. Heinz, <a href="/A224769/b224769_1.txt">Table of n, a(n) for n = 0..1000</a>
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Alois P. Heinz, <a href="/A224769/b224769_1.txt">Table of n, a(n) for n = 0..5001000</a>
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a(n) ~ c * d^n / n^(3/2), where d = 3/4*(71 + 8*sqrt(2))^(1/3) + 51/(4*(71 + 8*sqrt(2))^(1/3)) + 13/4 = 9.4435356015932520820011..., c = 0.00814413508604516738631686716788556507884786... . - Vaclav Kotesovec, Sep 07 2014
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