Pyramid #3
Pyramid #4
Pyramid #5
Generating function
$$U_{4}(x, y) = x y + x + y$$
Explicit formula
$$T_{4}(n, m, k) = {\binom{k}{m}} {\binom{m}{- k + m + n}}$$
Data table
0 1 0 0 0 0 0
1 1 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
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