A069828 Sum of positive integers k for k <= n and gcd(k,n) = gcd(k+1,n).

1, 0, 1, 0, 6, 0, 15, 0, 12, 0, 45, 0, 66, 0, 21, 0, 120, 0, 153, 0, 50, 0, 231, 0, 180, 0, 117, 0, 378, 0, 435, 0, 144, 0, 255, 0, 630, 0, 209, 0, 780, 0, 861, 0, 198, 0, 1035, 0, 840, 0, 375, 0, 1326, 0, 729, 0, 476, 0, 1653, 0, 1770, 0, 465, 0, 1056, 0, 2145, 0, 714, 0, 2415

Offset

1,5

Links

Table of n, a(n) for n=1..71.

Formula

a(n) = (n*(n-1)/2)*Sum_{d|n} mu(d)*tau(d)/d, n > 1.

Prog

(PARI) a(n) = if (n==1, 1, n*(n-1)/2*sumdiv(n, d, moebius(d)*numdiv(d)/d)) \\ Michel Marcus, Jun 17 2013

Crossrefs

Cf. A058026, A070555.

Sequence in context: A283999 A240813 A175567 * A340951 A270536 A278712

Adjacent sequences: A069825 A069826 A069827 * A069829 A069830 A069831

Keyword

nonn

Author

Vladeta Jovovic, Apr 29 2002

Status

approved