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%I #18 Feb 24 2024 01:06:08
%S 7,14,15,28,30,45,49,56,60,75,90,98,112,120,135,150,180,196,224,225,
%T 240,270,300,343,360,375,392,405,448,450,480,540,600,675,686,720,750,
%U 784,810,896,900,960,1080,1125,1200,1215,1350,1372,1440,1500,1568,1620
%N Numbers k such that k-th cyclotomic polynomial has exactly 7 nonzero terms.
%F {n: A051664(n)=7}. - _R. J. Mathar_, Sep 15 2012
%t Select[Range[2000], Count[CoefficientList[Cyclotomic[#, x], x], 0] == EulerPhi[#] - 6 &] (* _T. D. Noe_, Feb 13 2012 *)
%o (PARI) isok(n) = {my(p = polcyclo(n)); #select(x->x, vector(1+poldegree(p), k, polcoeff(p, k-1))) == 7;} \\ _Michel Marcus_, Oct 26 2017
%Y Cf. A086761.
%K nonn
%O 1,1
%A Yuval Dekel (dekelyuval(AT)hotmail.com), Aug 03 2003
%E Extended by _T. D. Noe_, Feb 13 2012
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