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A006495 Real part of (1+2i)^n, where i is sqrt(-1).
(Formerly M2880)
21

%I M2880

%S 1,1,-3,-11,-7,41,117,29,-527,-1199,237,6469,11753,-8839,-76443,

%T -108691,164833,873121,922077,-2521451,-9653287,-6699319,34867797,

%U 103232189,32125393,-451910159,-1064447283,130656229,5583548873

%N Real part of (1+2i)^n, where i is sqrt(-1).

%C Row sums of the Euler related triangle A117411. Partial sums are A006495. - _Paul Barry_, Mar 16 2006

%C Binomial transform of [1, 0, -4, 0, 16, 0, -64, 0, 256, 0, ...], i.e. powers of -4 with interpolated zeros. - _Philippe Deléham_, Dec 02 2008

%C The absolute values of these numbers are the odd numbers y such that x^2 + y^2 = 5^n with x and y coprime. See A098122. - _T. D. Noe_, Apr 14 2011

%C Pisano period lengths: 1, 1, 8, 1, 4, 8, 48, 4, 24, 4, 60, 8, 12, 48, 8, 8, 16, 24, 90, 4,... - _R. J. Mathar_, Aug 10 2012

%C Multiplied by a signed sequence of 2's we obtain 2, -2, -6, 22, -14, -82, 234, -58, -1054, 2398, 474, -12938,..., the Lucas V(-2,5) sequence. - _R. J. Mathar_, Jan 08 2013

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H Vincenzo Librandi, <a href="/A006495/b006495.txt">Table of n, a(n) for n = 0..200</a>

%H G. Berzsenyi, <a href="http://www.fq.math.ca/Scanned/15-3/berzsenyi.pdf">Gaussian Fibonacci numbers</a>, Fib. Quart., 15 (1977), 233-236.

%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Lucas_sequence#Specific_names">Lucas sequence</a>

%H <a href="/index/Lu#Lucas">Index entries for Lucas sequences</a>

%H <a href="/index/Ga#gaussians">Index entries for Gaussian integers and primes</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-5).

%F a(n) = (1/2)*((1+2*I)^n+(1-2*I)^n). - _Benoit Cloitre_, Oct 28 2002

%F G.f.: (1-x)/(1-2*x+5*x^2); a(n)=2*a(n-1)-5*a(n-2); a(n) = 5^(n/2)*cos(n*atan(1/3)+Pi*n/4); a(n) = Sum_{k=0..n} Sum_{j=0..n-k} C(n,k-j)*C(j,n-k)}*(-4)^(n-k). - _Paul Barry_, Mar 16 2006

%F A000351(n) = a(n)^2 + A006496(n)^2. - Fabrice Baubet (intih(AT)free.fr), May 28 2007

%F a(n) = upper left and lower right terms of the 2 X 2 matrix [1,-2; 2,1]^n. - _Gary W. Adamson_, Mar 28 2008

%F a(n) = Sum_{k=0..n} A124182(n,k)*(-5)^(n-k). - _Philippe Deléham_, Nov 01 2008

%F a(n) = Sum_{k=0..n} A098158(n,k)*(-4)^(n-k). - _Philippe Deléham_, Nov 14 2008

%F a(n) = (4*n+5)*a(n-1) - 8*Sum_{k=1..n} a(k-1)*a(n-k) if n>0. - _Michael Somos_, Jul 23 2011

%F E.g.f.: exp(x)*cos(2*x). - _Sergei N. Gladkovskii_, Jul 22 2012

%F a(n) = 5^(n/2) * cos(n*arctan(2)). - _Sergei N. Gladkovskii_, Aug 13 2012

%F G.f.: G(0)/2, where G(k)= 1 + 1/(1 - x*(4*k+1)/(x*(4*k+5) + 1/G(k+1))); (continued fraction). - _Sergei N. Gladkovskii_, May 26 2013

%e 1 + x - 3*x^2 - 11*x^3 - 7*x^4 + 41*x^5 + 117*x^6 + 29*x^7 - 527*x^8 + ...

%t Table[Re[(1+2I)^n],{n,0,29}] (* _Giovanni Resta_, Mar 28 2006 *)

%o (Sage) [lucas_number2(n,2,5)/2 for n in xrange(0,30)] # _Zerinvary Lajos_, Jul 08 2008

%o (MAGMA) A006495:=func< n | Integers()!Real((1+2*Sqrt(-1))^n) >; [ A006495(n): n in [0..30] ]; // _Klaus Brockhaus_, Feb 04 2011

%o (PARI) {a(n) = local(A); n++; if( n<1, 0, A = vector(n); A[1] = 1; for( k=2, n, A[k] = (4*k + 1) * A[k-1] - 8 * sum( j=1, k-1, A[j] * A[k-j])); A[n])} /* _Michael Somos_, Jul 23 2011 */

%o (PARI) a(n)=real((1+2*I)^n) \\ _Charles R Greathouse IV_, Nov 21 2014

%Y Cf. A006496, A045873 (partial sums).

%K sign,easy

%O 0,3

%A _N. J. A. Sloane_.

%E Signs from _Christian G. Bower_, Nov 15 1998

%E Corrected by _Giovanni Resta_, Mar 28 2006

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Last modified February 16 04:47 EST 2019. Contains 320140 sequences. (Running on oeis4.)