OFFSET
1,2
COMMENTS
Partial sums of a(n) give A015634(n).
See also A059358, A116963 (applied to shifted version of tetrahedral numbers), inverse Moebius transform of tetrahedral numbers. - Jonathan Vos Post, Apr 20 2006
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = |{(x,y,z) : 1 <= x <= y <= z <= n, gcd(x,y,z,n) = 1}|.
G.f.: Sum_{k>=1} mu(k) * x^k / (1 - x^k)^4. - Ilya Gutkovskiy, Feb 13 2020
EXAMPLE
a(2)=3 because of the triples (1,1,1), (1,1,2), (1,2,2).
PROG
(PARI) a(n) = sumdiv(n, d, binomial(d+2, 3)*moebius(n/d)); \\ Michel Marcus, Nov 04 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Steve Butler, Apr 18 2006
EXTENSIONS
Offset changed to 1 by Ilya Gutkovskiy, Feb 13 2020
STATUS
approved