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A309280
T(n,k) is (1/k) times the sum of the elements of all subsets of [n] whose sum is divisible by k; triangle T(n,k), n >= 1, 1 <= k <= n*(n+1)/2, read by rows.
13
1, 6, 1, 1, 24, 6, 4, 1, 1, 1, 80, 20, 9, 4, 4, 2, 2, 1, 1, 1, 240, 60, 30, 14, 12, 7, 5, 3, 3, 3, 2, 2, 1, 1, 1, 672, 168, 84, 42, 29, 20, 15, 10, 9, 7, 5, 5, 4, 4, 4, 3, 2, 2, 1, 1, 1, 1792, 448, 202, 112, 71, 49, 40, 27, 23, 17, 15, 12, 10, 10, 8, 8, 7, 7, 6, 5, 5, 4, 3, 2, 2, 1, 1, 1
OFFSET
1,2
COMMENTS
T(n,k) is defined for all n >= 0, k >= 1. The triangle contains only the positive terms. T(n,k) = 0 if k > n*(n+1)/2.
The sequence of column k satisfies a linear recurrence with constant coefficients of order 3*A000593(k).
LINKS
FORMULA
T(n+1,n*(n+1)/2+1) = A000009(n) for n >= 0.
EXAMPLE
The subsets of [4] whose sum is divisible by 3 are: {}, {3}, {1,2}, {2,4}, {1,2,3}, {2,3,4}. The sum of their elements is 0 + 3 + 3 + 6 + 6 + 9 = 27. So T(4,3) = 27/3 = 9.
Triangle T(n,k) begins:
1;
6, 1, 1;
24, 6, 4, 1, 1, 1;
80, 20, 9, 4, 4, 2, 2, 1, 1, 1;
240, 60, 30, 14, 12, 7, 5, 3, 3, 3, 2, 2, 1, 1, 1;
...
MAPLE
b:= proc(n, m, s) option remember; `if`(n=0, [`if`(s=0, 1, 0), 0],
b(n-1, m, s) +(g-> g+[0, g[1]*n])(b(n-1, m, irem(s+n, m))))
end:
T:= (n, k)-> b(n, k, 0)[2]/k:
seq(seq(T(n, k), k=1..n*(n+1)/2), n=1..10);
# second Maple program:
b:= proc(n, s) option remember; `if`(n=0, add(s/d *x^d,
d=numtheory[divisors](s)), b(n-1, s)+b(n-1, s+n))
end:
T:= n-> (p-> seq(coeff(p, x, i), i=1..degree(p)))(b(n, 0)):
seq(T(n), n=1..10);
MATHEMATICA
b[n_, m_, s_] := b[n, m, s] = If[n == 0, {If[s == 0, 1, 0], 0}, b[n-1, m, s] + Function[g, g + {0, g[[1]] n}][b[n-1, m, Mod[s+n, m]]]];
T[n_, k_] := b[n, k, 0][[2]]/k;
Table[T[n, k], {n, 1, 10}, {k, 1, n(n+1)/2}] // Flatten (* Jean-François Alcover, Oct 04 2019, after Alois P. Heinz *)
CROSSREFS
Row sums give A309281.
Row lengths give A000217.
T(n,n) gives A309128.
Rows reversed converge to A000009.
Sequence in context: A138076 A174527 A156139 * A155863 A173882 A174045
KEYWORD
nonn,look,tabf
AUTHOR
Alois P. Heinz, Jul 20 2019
STATUS
approved