OFFSET
1,2
COMMENTS
Definition 1.1 defines F_3^n as a Boolean function and definition 1.3 defines the Fourier transform of a Boolean function. - Michael Somos, Aug 04 2012
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..2500
X. Zhang, H. Guo, R. Feng and Y. Li, Proof of a conjecture about rotation symmetric functions, Discrete Math., 311 (2011), 1281-1289.
Index entries for linear recurrences with constant coefficients, signature (0,2,2).
FORMULA
a(n) = hat{F_3^n}(0), the Fourier transform evaluated at 0 of the Boolean function F_3^n defined by F_3^n(x_0, ..., x_{n-1}) = Sum_{ 0<i<=n-1} x_i x_{i+1(mod n)} x_{i+2(mod n)}. - Michael Somos, Aug 04 2012
From Michael Somos, Aug 04 2012: (Start)
G.f.: 2 * x^2 * (2 + 3*x) / (1 - 2*x^2 - 2*x^3).
a(n + 3) = 2*a(n + 1) + 2*a(n). (End)
EXAMPLE
G.f. = 4*x^2 + 6*x^3 + 8*x^4 + 20*x^5 + 28*x^6 + 56*x^7 + 96*x^8 + 168*x^9 + ...
MATHEMATICA
Rest[CoefficientList[Series[2*x^2*(2+3*x)/(1-2*x^2-2*x^3), {x, 0, 50}], x]] (* G. C. Greubel, Aug 13 2018 *)
LinearRecurrence[{0, 2, 2}, {0, 4, 6}, 40] (* Vincenzo Librandi, Aug 14 2018 *)
PROG
(PARI) {a(n) = if( n<1, 0, polsym( x^3 - 2*x - 2, n)[n + 1])}; /* Michael Somos, Aug 04 2012 */
(PARI) {a(n) = if( n<1, 0, sum( x=0, 2^n-1, (-1)^sum( i=0, n-1, bittest(x, i) * bittest(x, (i+1)%n) * bittest(x, (i+2)%n))))}; /* Michael Somos, Aug 04 2012 */
(Magma) m:=50; R<x>:=PowerSeriesRing(Integers(), m); [0] cat Coefficients(R!(2*x^2*(2+3*x)/(1-2*x^2-2*x^3))); // G. C. Greubel, Aug 13 2018
(Magma) I:=[0, 4, 6]; [n le 3 select I[n] else 2*Self(n-2)+2*Self(n-3): n in [1..50]]; // Vincenzo Librandi, Aug 14 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Jun 19 2011
STATUS
approved