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A281871
Number T(n,k) of k-element subsets of [n] having a square element sum; triangle T(n,k), n>=0, 0<=k<=n, read by rows.
25
1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 2, 1, 1, 0, 1, 2, 2, 2, 0, 0, 1, 2, 3, 3, 2, 1, 0, 1, 2, 4, 5, 5, 2, 1, 0, 1, 2, 5, 8, 8, 6, 3, 0, 1, 1, 3, 6, 11, 14, 13, 7, 4, 1, 0, 1, 3, 7, 15, 23, 24, 19, 10, 3, 1, 0, 1, 3, 8, 20, 34, 43, 39, 25, 13, 3, 1, 0, 1, 3, 9, 26, 49, 71, 74, 60, 34, 14, 5, 0, 0
OFFSET
0,12
LINKS
FORMULA
T(n,n) = 1 for n in { A001108 }, T(n,n) = 0 otherwise.
T(n,n-1) = 1 for n in { A214857 }, T(n,n-1) = 0 for n in { A214858 }.
Sum_{k=0..n} k * T(n,k) = A377572(n).
EXAMPLE
T(7,0) = 1: {}.
T(7,1) = 2: {1}, {4}.
T(7,2) = 4: {1,3}, {2,7}, {3,6}, {4,5}.
T(7,3) = 5: {1,2,6}, {1,3,5}, {2,3,4}, {3,6,7}, {4,5,7}.
T(7,4) = 5: {1,2,6,7}, {1,3,5,7}, {1,4,5,6}, {2,3,4,7}, {2,3,5,6}.
T(7,5) = 2: {1,2,3,4,6}, {3,4,5,6,7}.
T(7,6) = 1: {1,2,4,5,6,7}.
T(7,7) = 0.
T(8,8) = 1: {1,2,3,4,5,6,7,8}.
Triangle T(n,k) begins:
1;
1, 1;
1, 1, 0;
1, 1, 1, 0;
1, 2, 1, 1, 0;
1, 2, 2, 2, 0, 0;
1, 2, 3, 3, 2, 1, 0;
1, 2, 4, 5, 5, 2, 1, 0;
1, 2, 5, 8, 8, 6, 3, 0, 1;
1, 3, 6, 11, 14, 13, 7, 4, 1, 0;
1, 3, 7, 15, 23, 24, 19, 10, 3, 1, 0;
1, 3, 8, 20, 34, 43, 39, 25, 13, 3, 1, 0;
1, 3, 9, 26, 49, 71, 74, 60, 34, 14, 5, 0, 0;
...
MAPLE
b:= proc(n, s) option remember; expand(`if`(n=0,
`if`(issqr(s), 1, 0), b(n-1, s)+x*b(n-1, s+n)))
end:
T:= n-> (p-> seq(coeff(p, x, i), i=0..n))(b(n, 0)):
seq(T(n), n=0..16);
MATHEMATICA
b[n_, s_] := b[n, s] = Expand[If[n == 0, If[IntegerQ @ Sqrt[s], 1, 0], b[n - 1, s] + x*b[n - 1, s + n]]];
T[n_] := Function[p, Table[Coefficient[p, x, i], {i, 0, n}]][b[n, 0]];
Table[T[n], {n, 0, 10}] // Flatten (* Jean-François Alcover, Jun 03 2018, from Maple *)
CROSSREFS
Main diagonal is characteristic function of A001108.
Diagonals T(n+k,n) for k=2-10 give: A281965, A281966, A281967, A281968, A281969, A281970, A281971, A281972, A281973.
Row sums give A126024.
T(2n,n) gives A281872.
Sequence in context: A099314 A214341 A381104 * A131334 A004602 A247418
KEYWORD
nonn,tabl
AUTHOR
Alois P. Heinz, Jan 31 2017
STATUS
approved