OFFSET
1,2
COMMENTS
Partial sums of a(n) give A015650(n).
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = |{(x,y,z,w) : 1 <= x <= y <= z <= w <= n, gcd(x,y,z,w,n) = 1}|.
G.f.: Sum_{k>=1} mu(k) * x^k / (1 - x^k)^5. - Ilya Gutkovskiy, Feb 13 2020
EXAMPLE
a(2)=4 because of the quadruples (1,1,1,1), (1,1,1,2), (1,1,2,2), (1,2,2,2).
MAPLE
b34:= unapply(expand(binomial(n+3, 4)), n):
f:= proc(n) local k; uses numtheory;
add(b34(k)*mobius(n/k), k=divisors(n))
end proc:
map(f, [$1..100]); # Robert Israel, May 24 2019
MATHEMATICA
a[n_] := Sum[Binomial[k+3, 4] MoebiusMu[n/k], {k, Divisors[n]}];
Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Feb 01 2023 *)
PROG
(PARI) a(n) = sumdiv(n, k, binomial(k+3, 4)*moebius(n/k)); \\ Michel Marcus, Nov 04 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Steve Butler, Apr 18 2006
STATUS
approved