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T[n_, k_] := Sum[Binomial[k, 3j] Binomial[n-k, 3j], {j, 0, n-k}];
Table[T[n, k], {n, 0, 11}, {k, 0, n}] // Flatten (* Jean-François Alcover, Sep 14 2023 *)
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Column k has g.f. (x^k/(1-x)) *sum Sum_{j=0..k, } C(k,3j)(x/(1-x))^(3j)}.
Number triangle T(n,k) =sum Sum_{j=0..n-k, } C(k,3j)*C(n-k,3j)}.
Seiichi Manyama, <a href="/A119335/b119335.txt">Table of n, a(n) for Rows n = 0..9869139, flattened</a>
Seiichi Manyama, <a href="/A119335/b119335.txt">Table of n, a(n) for n = 0..9869</a>
More terms from Seiichi Manyama, Mar 12 2019