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 A089128 a(n) = gcd(6,n). 12
 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Also greatest common divisor of n^2-6 and n^2+6. The second term of sequences of this type for n=0,1,2... form the sequence 1,2,1,2,1,... in decimal 0.1212121212... = 4/33. Multiplicative with a(p^e) = GCD(p^e, 6). - David W. Wilson, Jun 12 2005 From Jaroslav Krizek, May 27 2010: (Start) a(n) = denominators of averages of squares of the first n positive integers for n >= 1. a(n) is periodic sequence with period (6, 1, 2, 3, 2, 1). See A175485 - numerators of averages of squares of the first n positive integers. a(n) = A175485(n) * n / A000330(n). For n = 337 holds: a(n) = 1 and simultaneously A175485(n) is square ( = 38025 = 195^2), i.e., the number k = 195 is quadratic mean (root mean square) of the first 337 positive integers. There are other such numbers - see A084231 and A084232. Sqrt(A175485(n) / a(n)) for n >= 1 is the harmonic mean of the first n positive integers. (End) LINKS Index entries for linear recurrences with constant coefficients, signature (0,0,0,0,0,1) FORMULA a(n) = 1 + [2|n] + 2*[3|n] + 2*[6|n] = (15 + 5*(-1)^n + 4*cos(n*Pi/3) + 12*cos(n*2*Pi/3))/6, where [x|y] is 1 if x divides y, 0 otherwise. - Mitch Harris Jun 15 2005 a(n) = (-2*(n mod 6) + (n+1 mod 6) + (n+2 mod 6) + 3*(n+5 mod 6))/3 (cf. forms of modular arithmetic of Paolo P. Lava, i.e., see A146094). - Bruno Berselli, Sep 27 2010 From R. J. Mathar, Apr 04 2011: (Start) Dirichlet g.f.: zeta(s)*(1+1/2^s+2/3^s+2/6^s). G.f.: (-6 - x - 2*x^2 - 3*x^3 - 2*x^4 - x^5) / ((x-1)*(1+x)*(1 + x + x^2)*(x^2 - x + 1)). (End) a(n) = ((n-1) mod 2 + 1)*( 2*floor(((n-1) mod 3)/2) + 1). - Gary Detlefs, Dec 28 2011 MATHEMATICA GCD[Range[0, 110], 6] (* or *) PadRight[{}, 110, {6, 1, 2, 3, 2, 1}] (* Harvey P. Dale, Dec 26 2018 *) PROG (PARI) g(n) = for(x=0, n, print1(gcd(x^2-6, x^2+6)", ")) (Haskell) a089128 = gcd 6  -- Reinhard Zumkeller, Apr 06 2015 CROSSREFS Sequence in context: A165070 A246711 A164809 * A222215 A106687 A083463 Adjacent sequences:  A089125 A089126 A089127 * A089129 A089130 A089131 KEYWORD easy,nonn,mult AUTHOR Cino Hilliard, Dec 05 2003 EXTENSIONS Name changed, using David W. Wilson's formula, by Franklin T. Adams-Watters, May 16 2018 STATUS approved

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Last modified February 22 07:46 EST 2020. Contains 332118 sequences. (Running on oeis4.)