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 A080278 a(n) = (3^(v_3(n) + 1) - 1)/2, where v_3(n) = highest power of 3 dividing n = A007949(n). 12
 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 40, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 40, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1, 121, 1, 1, 4, 1, 1, 4, 1, 1, 13, 1, 1, 4, 1, 1, 4, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Denominator of the quotient sigma(3*n)/sigma(n). - Labos Elemer, Nov 04 2003 a(n) = b/(3*(c+d)) where b, c, d are the sums of the divisors of 3*n that are congruent respectively to 0, 1 and 2 mod 3. - Michel Lagneau, Nov 05 2012 Sum of powers of 3 dividing n. - Amiram Eldar, Nov 27 2022 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 Klaus Brockhaus, Illustration of A080278 and A080333 FORMULA G.f.: Sum_{k>=0} 3^k*x^(3^k)/(1-x^(3^k)). - Ralf Stephan, Jun 15 2003 L.g.f.: -log(Product_{k>=0} (1 - x^(3^k))) = Sum_{n>=1} a(n)*x^n/n. - Ilya Gutkovskiy, Mar 15 2018 a(n) = sigma(n)/(sigma(3*n) - 3*sigma(n)), where sigma(n) = A000203(n). - Peter Bala, Jun 10 2022 From Amiram Eldar, Nov 27 2022: (Start) Multiplicative with a(3^e) = (3^(e+1)-1)/2, and a(p^e) = 1 for p != 3. Dirichlet g.f.: zeta(s) / (1 - 3^(1 - s)). Sum_{k=1..n} a(k) ~ n*log_3(n) + (1/2 + (gamma - 1)/log(3))*n, where gamma is Euler's constant (A001620). (End) EXAMPLE a(6) = 4 because the divisors of 3*6 = 18 are {1, 2, 3, 6, 9, 18} => b = 3 + 6 + 9 + 18 = 36, c = 1, d = 2, hence a(6) = b/(3*(c+d)) = 36/(3*(1+2)) = 36/9 = 4. - Michel Lagneau, Nov 05 2012 MAPLE A080278 := n->(3^(A007949(n)+1)-1)/2; MATHEMATICA Table[Denominator[DivisorSigma[1, 3*n]/DivisorSigma[1, n]], {n, 1, 128}] a[n_] := (3^(IntegerExponent[n, 3] + 1) - 1)/2; Array[a, 100] (* Amiram Eldar, Nov 27 2022 *) PROG (PARI) a(n) = denominator(sigma(3*n)/sigma(n)); \\ Michel Marcus, Dec 15 2019 (PARI) a(n) = (3^(valuation(n, 3) + 1) - 1)/2; \\ Amiram Eldar, Nov 27 2022 CROSSREFS Cf. A000203, A001620, A007949, A080333, A088838 (numerator of sigma(3*n)/sigma(n)). Sequence in context: A177023 A214333 A343408 * A258328 A070085 A131776 Adjacent sequences: A080275 A080276 A080277 * A080279 A080280 A080281 KEYWORD nonn,mult,frac AUTHOR N. J. A. Sloane, Mar 19 2003 STATUS approved

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Last modified June 24 00:02 EDT 2024. Contains 373661 sequences. (Running on oeis4.)