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A288852
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Number T(n,k) of matchings of size k in the n X n X n triangular grid; triangle T(n,k), n>=0, 0<=k<=floor(n*(n+1)/4), read by rows.
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6
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1, 1, 1, 3, 1, 9, 15, 2, 1, 18, 99, 193, 108, 6, 1, 30, 333, 1734, 4416, 5193, 2331, 240, 1, 45, 825, 8027, 45261, 151707, 298357, 327237, 180234, 40464, 2238, 1, 63, 1710, 26335, 255123, 1629474, 6995539, 20211423, 38743020, 47768064, 35913207, 15071019
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OFFSET
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0,4
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COMMENTS
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The n X n X n triangular grid has n rows with i vertices in row i. Each vertex is connected to the neighbors in the same row and up to two vertices in each of the neighboring rows. The graph has A000217(n) vertices and 3*A000217(n-1) edges altogether.
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LINKS
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FORMULA
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T(n,floor(n*(n+1)/4)) = A271610(n).
Sum_{i=0..1} T(n,floor(n*(n+1)/4)-i) = A271612(n).
Sum_{i=0..2} T(n,floor(n*(n+1)/4)-i) = A271614(n).
Sum_{i=0..3} T(n,floor(n*(n+1)/4)-i) = A271616(n).
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EXAMPLE
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Triangle T(n,k) begins:
1;
1;
1, 3;
1, 9, 15, 2;
1, 18, 99, 193, 108, 6;
1, 30, 333, 1734, 4416, 5193, 2331, 240;
1, 45, 825, 8027, 45261, 151707, 298357, 327237, 180234, 40464, 2238;
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MAPLE
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b:= proc(l) option remember; local n, k; n:= nops(l);
if n=0 then 1
elif min(l)>0 then b(subsop(-1=NULL, map(h-> h-1, l)))
else for k to n while l[k]>0 do od; b(subsop(k=1, l))+
expand(x*(`if`(k<n, b(subsop(k=2, l)), 0)+
`if`(k<n and l[k+1]=0, b(subsop(k=1, k+1=1, l)), 0)+
`if`(k>1 and l[k-1]=1, b(subsop(k=1, k-1=2, l)), 0)))
fi
end:
T:= n-> (p-> seq(coeff(p, x, i), i=0..degree(p)))(b([0$n])):
seq(T(n), n=0..10);
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MATHEMATICA
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b[l_] := b[l] = Module[{n = Length[l], k}, Which[n == 0, 1, Min[l] > 0, b[ReplacePart[l - 1, -1 -> Nothing]], True, For[k = 1, k <= n && l[[k]] > 0, k++]; b[ReplacePart[l, k -> 1]] + x*Expand[If[k < n, b[ReplacePart[l, k -> 2]], 0] + If[k < n && l[[k + 1]] == 0, b[ReplacePart[l, {k -> 1, k + 1 -> 1}]], 0] + If[k > 1 && l[[k - 1]] == 1, b[ReplacePart[l, {k -> 1, k - 1 -> 2}]], 0]]]];
T[n_] := Table[Coefficient[#, x, i], {i, 0, Exponent[#, x]}]&[b[Table[0, n] ]];
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CROSSREFS
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Last elements of rows give A271610.
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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