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 A006496 Imaginary part of (1+2i)^n. (Formerly M0933) 7
 0, 2, 4, -2, -24, -38, 44, 278, 336, -718, -3116, -2642, 10296, 33802, 16124, -136762, -354144, -24478, 1721764, 3565918, -1476984, -20783558, -34182196, 35553398, 242017776, 306268562, -597551756, -2726446322, -2465133864, 8701963882, 29729597084, 15949374758 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The absolute values of these numbers are the even numbers x such that x^2 + y^2 = 5^n with x and y coprime. See A098122. - T. D. Noe, Apr 14 2011 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..200 G. Berzsenyi, Gaussian Fibonacci numbers, Fib. Quart., 15 (1977), 233-236. Index entries for linear recurrences with constant coefficients, signature (2,-5). FORMULA a(0) = 0, a(1) = 2, a(n) = 2a(n-1)-5a(n-2). - T. D. Noe, Nov 09 2006 For all n, a(n) = - [M^n]_1,2, where M = [1, -2; 2, 1]. - Simone Severini, Apr 25 2007 A000351(n) = A006495(n)^2 + a(n)^2. - Fabrice Baubet (intih(AT)free.fr), May 28 2007 O.g.f.: 2*x/(1-2*x+5*x^2). a(n)=2*A045873(n). - R. J. Mathar, Apr 06 2008 a(n)=(1/2)*I*(1-2*I)^n-(1/2)*I*(1+2*I)^n, with n>=0 and I=sqrt(-1). - Paolo P. Lava, Oct 03 2008 E.g.f.: exp(x)*sin(2*x). - Sergei N. Gladkovskii, Jul 22 2012 MATHEMATICA LinearRecurrence[{2, -5}, {0, 2}, 30] (* Vincenzo Librandi, Dec 21 2011 *) PROG (MAGMA) I:=[0, 2]; [n le 2 select I[n] else 2*Self(n-1)-5*Self(n-2): n in [1..40]]; // Vincenzo Librandi, Dec 21 2011 (PARI) a(n)=([1, -2; 2, 1]^n)[1, 2] \\ Charles R Greathouse IV, Dec 22 2011 CROSSREFS Cf. A006495. Sequence in context: A100944 A059890 A210457 * A263931 A259685 A130172 Adjacent sequences:  A006493 A006494 A006495 * A006497 A006498 A006499 KEYWORD sign,easy AUTHOR EXTENSIONS Signs from Christian G. Bower, Nov 15 1998 Corrected by T. D. Noe, Nov 09 2006 More terms from R. J. Mathar, Apr 06 2008 STATUS approved

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