A241565 Number of 3-element subsets of {1,...,n} whose sum has more than 4 divisors.

0, 0, 0, 0, 1, 3, 8, 15, 27, 38, 54, 71, 95, 119, 152, 186, 230, 277, 338, 404, 486, 571, 672, 777, 901, 1028, 1175, 1327, 1500, 1679, 1881, 2090, 2322, 2559, 2822, 3092, 3389, 3693, 4026, 4366, 4735, 5110, 5515, 5928, 6376, 6831, 7322, 7823, 8362, 8909

Offset

1,6

Comments

If the constraint on the number of divisors is dropped, one gets A000292 = tetrahedral numbers C(n+2,3) = n*(n+1)*(n+2)/6, which therefore is an upper bound.

If the sums are required to have more than 3 divisors, one gets A241564.

Links

Table of n, a(n) for n=1..50.

Prog

(PARI) a(n, m=3, d=4)={s=0; u=vector(m, n, 1)~; forvec(v=vector(m, i, [1, n]), numdiv(v*u)>d&&s++, 2); s}

Crossrefs

Sequence in context: A172296 A048598 A049717 * A359210 A047866 A080183

Adjacent sequences: A241562 A241563 A241564 * A241566 A241567 A241568

Keyword

nonn

Author

M. F. Hasler, Apr 25 2014

Status

approved