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%I #25 Sep 08 2022 08:44:39
%S 1,1,1,0,0,0,0,-1,-1,-1,0,0,0,0,0,0,0,0,0,0,0,1,1,1,0,0,0,0,-1,-1,-1,
%T 0,0,0,0,0,0,0,0,0,0,0,1,1,1,0,0,0,0,-1,-1,-1,0,0,0,0,0,0,0,0,0,0,0,1,
%U 1,1,0,0,0,0,-1,-1,-1,0,0,0,0,0,0,0,0
%N Inverse of 21st cyclotomic polynomial.
%C Periodic with period length 21. - _Ray Chandler_, Apr 03 2017
%H Vincenzo Librandi, <a href="/A014030/b014030.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_12">Index entries for linear recurrences with constant coefficients</a>, signature (1, 0, -1, 1, 0, -1, 0, 1, -1, 0, 1, -1).
%H <a href="/index/Pol#poly_cyclo_inv">Index to sequences related to inverse of cyclotomic polynomials</a>
%F G.f.: 1/(1 - x + x^3 - x^4 + x^6 - x^8 + x^9 - x^11 + x^12). - _Ilya Gutkovskiy_, Aug 18 2017
%p with(numtheory,cyclotomic); c := n->series(1/cyclotomic(n,x),x,120); seq(coeff(c(21), x, n), n=0..100);
%t CoefficientList[Series[1/Cyclotomic[21, x], {x, 0, 100}], x] (* _Vincenzo Librandi_, Apr 03 2014 *)
%t LinearRecurrence[{1, 0, -1, 1, 0, -1, 0, 1, -1, 0, 1, -1},{1, 1, 1, 0, 0, 0, 0, -1, -1, -1, 0, 0},81] (* _Ray Chandler_, Sep 15 2015 *)
%o (PARI) Vec(1/polcyclo(21)+O(x^99)) \\ _Charles R Greathouse IV_, Mar 24 2014
%o (Magma) &cat[[1,1,1,0,0,0,0,-1,-1,-1,0,0,0,0,0,0,0,0,0, 0,0]: n in [0..6]]; // _Vincenzo Librandi_, Apr 03 2014
%K sign,easy
%O 0,1
%A _Simon Plouffe_
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