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 A059973 Expansion of x*(1 + x - 2*x^2) / ( 1 - 4*x^2 - x^4). 7
 0, 1, 1, 2, 4, 9, 17, 38, 72, 161, 305, 682, 1292, 2889, 5473, 12238, 23184, 51841, 98209, 219602, 416020, 930249, 1762289, 3940598, 7465176, 16692641, 31622993, 70711162, 133957148, 299537289, 567451585, 1268860318, 2403763488, 5374978561 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Based on fact that cube root of (2 +- 1 sqrt(5)) = sixth root of (9 +- 4 sqrt(5)) = ninth root of (38 +- 17 sqrt(5)) = ... = phi or 1/phi, where phi is the golden ratio. Osler gives the first three of the above equalities with phi on page 27, stating they are simplified expressions from Ramanujan, but without hinting that the series continues. Bisections: A001076 and A001077. LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 T. J. Osler, Cardan polynomials and the reduction of radicals, Math. Mag., 74 (No. 1, 2001), 26-32. Index entries for linear recurrences with constant coefficients, signature (0,4,0,1). FORMULA From Michael Somos, Aug 11 2009: (Start) a(2*n) = A001076(n). a(2*n+1) = A001077(n). (End) Recurrence: a(n) = 4*a(n-2) + a(n-4) for n >= 4; a(0)=0, a(1)=a(2)=1, a(3)=2. - Werner Schulte, Oct 03 2015 From Altug Alkan, Oct 06 2015: (Start) a(2n) = Sum_{k=0..2n-1} a(k). a(2n+1) = A001076(n-1) + Sum_{k=0..2n} a(k), n>0. (End) EXAMPLE G.f. = x + x^2 + 2*x^3 + 4*x^4 + 9*x^5 + 17*x^6 + 38*x^7 + 72*x^8 + 161*x^9 + ... - Michael Somos, Aug 11 2009 MATHEMATICA CoefficientList[ Series[(x +x^2 -2x^3)/(1 -4x^2 -x^4), {x, 0, 33}], x] LinearRecurrence[{0, 4, 0, 1}, {0, 1, 1, 2}, 50] (* Vincenzo Librandi, Oct 10 2015 *) PROG (PARI) {a(n) = if( n<0, n = -n; polcoeff( (-2*x + x^2 + x^3) / (1 + 4*x^2 - x^4) + x*O(x^n), n), polcoeff( (x + x^2 - 2*x^3) / ( 1 - 4*x^2 - x^4) + x*O(x^n), n))} /* Michael Somos, Aug 11 2009 */ (PARI) a(n) = if (n < 4, fibonacci(n), 4*a(n-2) + a(n-4)); vector(50, n, a(n-1)) \\ Altug Alkan, Oct 04 2015 (Magma) I:=[0, 1, 1, 2]; [n le 4 select I[n] else 4*Self(n-2)+Self(n-4): n in [1..40]]; // Vincenzo Librandi, Oct 10 2015 (Sage) def a(n): return fibonacci(n) if (n<4) else 4*a(n-2) + a(n-4) [a(n) for n in [0..40]] # G. C. Greubel, Jul 12 2021 CROSSREFS Cf. A000045, A001076, A001077. Cf. A179319, A183555, A183556. Sequence in context: A268649 A316983 A136326 * A030035 A123431 A049961 Adjacent sequences:  A059970 A059971 A059972 * A059974 A059975 A059976 KEYWORD easy,nonn,changed AUTHOR H. Peter Aleff (hpaleff(AT)earthlink.net), Mar 05 2001 EXTENSIONS Edited by Randall L Rathbun, Jan 11 2002 More terms from Sascha Kurz, Jan 31 2003 I made the old definition into a comment and gave the g.f. as an explicit definition. - N. J. A. Sloane, Jan 05 2011 Moved g.f. from Michael Somos, into name to match terms. - Paul D. Hanna, Jan 12 2011 STATUS approved

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Last modified July 5 20:09 EDT 2022. Contains 355102 sequences. (Running on oeis4.)