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A015631 Number of ordered triples of integers from [ 1..n ] with no global factor. 8
1, 3, 8, 15, 29, 42, 69, 95, 134, 172, 237, 287, 377, 452, 552, 652, 804, 915, 1104, 1252, 1450, 1635, 1910, 2106, 2416, 2674, 3007, 3301, 3735, 4027, 4522, 4914, 5404, 5844, 6432, 6870, 7572, 8121, 8805, 9389, 10249, 10831, 11776, 12506 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Number of integer-sided triangles with at least two sides <= n and sides relatively prime. - Henry Bottomley, Sep 29 2006

LINKS

R. J. Mathar, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = (A071778(n)+3*A018805(n)+2)/6. - Vladeta Jovovic, Dec 01 2004

Partial sums of the Moebius transform of the triangular numbers (A007438) - Steve Butler, Apr 18 2006

a(n) = 2*A123324(n) - A046657(n) for n>1. - Henry Bottomley, Sep 29 2006

Row sums of triangle A134543 - Gary W. Adamson, Oct 31 2007

EXAMPLE

a(4) = 15 because the 15 triplets in question are in lexicographic order: [1, 1, 1], [1, 1, 2], [1, 1, 3], [1, 1, 4], [1, 2, 2], [1, 2, 3], [1, 2, 4], [1, 3, 3], [1, 3, 4], [1, 4, 4], [2, 2, 3], [2, 3, 3], [2, 3, 4], [3, 3, 4] and [3, 4, 4]. - Wolfdieter Lang, Apr 04 2013

MAPLE

with(numtheory):

b:= proc(n) option remember;

       add(mobius(n/d)*d*(d+1)/2, d=divisors(n))

    end:

a:= proc(n) option remember;

      b(n) + `if`(n=1, 0, a(n-1))

    end:

seq(a(n), n=1..60);  # Alois P. Heinz, Feb 09 2011

MATHEMATICA

a[1] = 1; a[n_] := a[n] = Sum[MoebiusMu[n/d]*d*(d+1)/2, {d, Divisors[n]}] + a[n-1]; Table[a[n], {n, 1, 60}] (* Jean-François Alcover, Jan 20 2014, after Maple *)

CROSSREFS

Cf. A015616.

Cf. A134543.

Sequence in context: A036419 A054107 A097589 * A116686 A135350 A068038

Adjacent sequences:  A015628 A015629 A015630 * A015632 A015633 A015634

KEYWORD

nonn

AUTHOR

Olivier Gérard

STATUS

approved

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Last modified March 25 19:41 EDT 2017. Contains 284082 sequences.