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 A190914 Expansion of ( 5-9*x^2-2*x^3 ) / ( (1+x-x^2)*(1-x-x^2-x^3) ). 2
 5, 0, 6, 3, 18, 10, 57, 42, 178, 165, 566, 616, 1821, 2236, 5914, 7963, 19362, 27982, 63813, 97394, 211458, 336633, 703786, 1157544, 2350597, 3964960, 7872702, 13541691, 26425522, 46147178, 88853297, 156994354, 299165378, 533410837, 1008343310, 1810544592, 3401446413, 6140811708, 11481472994, 20815538227 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS The sequence ..., 14, 29, 10, 2, 9, 2, 0, [5], 0, 6, 3, 18, 10, 57, 42, ... (the number in square brackets at index 0) equals the trace of: [ 0 0 0 0-1 ] [ 1 0 0 0 0 ] [ 0 1 0 0 1 ]^(+n) [ 0 0 1 0 3 ] [ 0 0 0 1 0 ] or [ 0 0 0 0-1 ] [ 1 0 0 0 0 ] [ 0 1 0 0 3 ]^(-n) [ 0 0 1 0 1 ] [ 0 0 0 1 0 ] Its characteristic polynomial is (x^2 +/- x - 1) * (x^3 -/+ x^2 -/+ x - 1); these factors are fibonacci and tribonacci polynomials.  The ratio of negative terms approaches the golden ratio; the ratio of positive terms approaches the tribonacci constant. Prime numbers p divide a(+p) and a(-p), as the trace of a matrix M^p (mod p) is constant. Nonprimes c very rarely divide a(+c) and a(-c) simultaneously.  The only known dual pseudoprime in the sequence is 1. The distribution of residues induces gaps between pseudoprimes having roughly the size of c.  For example, after 1034881 there is a gap of more than one million terms without either variety of pseudoprime. Pseudoprimes appear limited to squared primes and squarefree numbers with three or more prime factors.  11 and 13 are more common than other factors. Positive pseudoprimes: c | a(+c) ---------------------------------------------- 1 3481. . . . 59^2 17143 . . . 7 31 79 105589. . . 11 29 331 635335. . . 5 283 449 2992191 . . 3 29 163 211 3659569 . . 1913^2 Negative pseudoprimes: c | a(-c) ---------------------------------------------- 1 9 . . . . . 3^2 806 . . . . 2 13 31 1419. . . . 3 11 43 6241. . . . 79^2 6721. . . . 11 13 47 12749 . . . 11 19 61 21106 . . . 2 61 173 38714 . . . 2 13 1489 146689. . . 383^2 649621. . . 7 17 53 103 1034881 . . 41 43 587 LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (0,3,1,0,-1). FORMULA a(n) = A061084(n+1) + A001644(n). - R. J. Mathar, Jun 06 2011 MATHEMATICA LinearRecurrence[{0, 3, 1, 0, -1}, {5, 0, 6, 3, 18}, 40] (* G. C. Greubel, Apr 23 2019 *) PROG (PARI) my(x='x+O('x^40)); Vec((5-9*x^2-2*x^3)/((1+x-x^2)*(1-x-x^2-x^3))) \\ G. C. Greubel, Apr 23 2019 (MAGMA) R:=PowerSeriesRing(Integers(), 40); Coefficients(R!( (5-9*x^2 -2*x^3)/((1+x-x^2)*(1-x-x^2-x^3)) )); // G. C. Greubel, Apr 23 2019 (Sage) ((5-9*x^2-2*x^3)/((1+x-x^2)*(1-x-x^2-x^3))).series(x, 40).coefficients(x, sparse=False) # G. C. Greubel, Apr 23 2019 CROSSREFS Cf. A190913 (extended to negative indices), A000045, A000073, A001608, A000040, A005117, A125666. Sequence in context: A035550 A197508 A159751 * A153458 A096287 A240243 Adjacent sequences:  A190911 A190912 A190913 * A190915 A190916 A190917 KEYWORD nonn AUTHOR Reikku Kulon, May 23 2011 STATUS approved

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Last modified September 24 08:31 EDT 2021. Contains 347623 sequences. (Running on oeis4.)