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 A261671 If n even, a(n) = 6n+3, otherwise a(n) = n. 4
 3, 1, 15, 3, 27, 5, 39, 7, 51, 9, 63, 11, 75, 13, 87, 15, 99, 17, 111, 19, 123, 21, 135, 23, 147, 25, 159, 27, 171, 29, 183, 31, 195, 33, 207, 35, 219, 37, 231, 39, 243, 41, 255, 43, 267, 45, 279, 47, 291, 49, 303, 51, 315, 53, 327, 55, 339, 57, 351, 59, 363 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 REFERENCES Although the OEIS indexes sequences by consecutive integers, it is better to think of this sequence as defined on the odd numbers 1,3,5,7,... and given by f(4m+1)=12m+3, f(4m+3)=2m+1, that is, subtract 1 and divide by 2 if the result is odd, otherwise multiply by 3. This arises in analyzing A109732. It is conjectured that starting with any positive odd number d and iterating f, we always eventually reach either 1, 9, or 33 (see Comments in A135730). LINKS Colin Barker, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (0,2,0,-1). FORMULA From Colin Barker, Sep 13 2015: (Start) a(n) = (3+3*(-1)^n+7*n+5*(-1)^n*n)/2. a(n) = 2*a(n-2) - a(n-4) for n>3. G.f.: (x^3+9*x^2+x+3) / ((x-1)^2*(x+1)^2). (End) PROG (PARI) Vec((x^3+9*x^2+x+3)/((x-1)^2*(x+1)^2) + O(x^100)) \\ Colin Barker, Sep 13 2015 CROSSREFS Cf. A109732, A135730. Sequence in context: A134145 A134146 A085569 * A198148 A286674 A286024 Adjacent sequences:  A261668 A261669 A261670 * A261672 A261673 A261674 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Sep 02 2015 STATUS approved

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Last modified March 23 02:42 EDT 2019. Contains 321422 sequences. (Running on oeis4.)