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 A069828 Sum of positive integers k for k <= n and gcd(k,n) = gcd(k+1,n). 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 LINKS 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 * A270536 A278712 A057401 Adjacent sequences:  A069825 A069826 A069827 * A069829 A069830 A069831 KEYWORD nonn AUTHOR Vladeta Jovovic, Apr 29 2002 STATUS approved

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Last modified December 13 17:17 EST 2019. Contains 329970 sequences. (Running on oeis4.)