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 A047789 Denominators of Glaisher's I-numbers. 6
 2, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 27, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 81, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 27, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 9, 1, 1, 3, 1, 1, 3, 1, 1, 27, 1, 1, 3, 1, 1, 3, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS Robert Israel, Table of n, a(n) for n = 0..10000 J. W. L. Glaisher, On a set of coefficients analogous to the Eulerian numbers, Proc. London Math. Soc., 31 (1899), 216-235. FORMULA From Robert Israel, Aug 14 2018: (Start) For n >= 1, a(3*n) = a(3*n+2) = 1 and a(3*n+1) = 3*a(n). G.f. g(x) satisfies g(x) = 3*x*g(x^3) + 2 - 3*x + (x^2+x^3)/(1-x^3). (End) EXAMPLE 1/2, 1/3, 1, 7, 809/9, 1847, 55601, 6921461/3,... MAPLE f:= n -> 3^padic:-ordp(2*n+1, 3): f(0):= 2: map(f, [\$0..200]); # Robert Israel, Aug 14 2018 MATHEMATICA a[0] = 2; a[n_] := 3^IntegerExponent[2n+1, 3]; Table[a[n], {n, 0, 101}] (* Jean-François Alcover, Feb 27 2019 *) PROG (PARI) a(n)=if(n<1, 2*(n==0), 3^valuation(2*n+1, 3)) /* Michael Somos, Feb 26 2004 */ (PARI) a(n)=if(n<1, 2*(n==0), n*=2; denominator(n!*polcoeff(3/(2+4*cos(x+O(x^n))), n))) /* Michael Somos, Feb 26 2004 */ CROSSREFS Cf. A047788, A002111. Sequence in context: A217897 A135900 A173272 * A068869 A251046 A064529 Adjacent sequences:  A047786 A047787 A047788 * A047790 A047791 A047792 KEYWORD nonn,frac AUTHOR STATUS approved

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Last modified March 22 12:46 EDT 2019. Contains 321421 sequences. (Running on oeis4.)