A241566 Number of 2-element subsets of {1,...,n} whose sum has more than 2 divisors.

0, 0, 1, 2, 5, 8, 12, 17, 22, 27, 34, 41, 50, 60, 70, 80, 92, 105, 119, 134, 149, 164, 181, 198, 216, 235, 254, 274, 296, 318, 341, 365, 390, 415, 441, 467, 494, 522, 551, 580, 611, 642, 675, 709, 743, 778, 815, 853, 891, 930, 969, 1008, 1049, 1090, 1131

Offset

1,4

Comments

If the constraint on the number of divisors is dropped, one gets A000217 = triangular numbers n*(n+1)/2, which therefore is an upper bound.

If one considers 3-element subsets instead, one gets A241563.

Links

Table of n, a(n) for n=1..55.

Prog

(PARI) a(n, m=2, d=2)={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: A036789 A330188 A214047 * A002960 A022942 A183861

Adjacent sequences: A241563 A241564 A241565 * A241567 A241568 A241569

Keyword

nonn

Author

M. F. Hasler, Apr 25 2014

Status

approved