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A059973 Expansion of (x + x^2 - 2*x^3) / ( 1 - 4*x^2 - x^4). 6
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

Table of n, a(n) for n=0..33.

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

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

a(2n) = Sum_{k=0..2n-1} a(k); a(2n+1) = A001076(n-1) + Sum_{k=0..2n} a(k), n>0. - Altug Alkan, Oct 06 2015

EXAMPLE

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 - 2 x^3)/(1 - 4 x^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

CROSSREFS

Cf. A000045 (Fibonacci Numbers).

A001076(n) = a(2*n), A001077(n) = a(2*n + 1). - Michael Somos, Aug 11 2009

Cf. A179319, A183555, A183556.

Sequence in context: A245122 A268649 A136326 * A030035 A123431 A283315

Adjacent sequences:  A059970 A059971 A059972 * A059974 A059975 A059976

KEYWORD

easy,nonn

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 March 24 15:46 EDT 2017. Contains 283991 sequences.