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%I #31 Feb 23 2024 07:28:09
%S 5,10,20,25,40,50,80,100,125,160,200,250,320,400,500,625,640,800,1000,
%T 1250,1280,1600,2000,2500,2560,3125,3200,4000,5000,5120,6250,6400,
%U 8000,10000,10240,12500,12800,15625,16000,20000,20480,25000,25600,31250,32000
%N Numbers k such that k-th cyclotomic polynomial has exactly 5 nonzero terms.
%C A206787(a(n)) = 6. - _Reinhard Zumkeller_, Feb 12 2012
%C All terms have the form 2^a 5^b with a >= 0 and b > 0. - _T. D. Noe_, Feb 13 2012
%C If the above holds for all terms then this sequence is 5 * A003592. - _David A. Corneth_, Jul 04 2018
%t Select[Range[1000], Count[CoefficientList[Cyclotomic[#, x], x], 0] == EulerPhi[#] - 4 &] (* _T. D. Noe_, Feb 13 2012 *)
%o (PARI) is(n) = v = Vec(polcyclo(n)); sum(i=1,#v,v[i]!=0) == 5 \\ _David A. Corneth_, Jul 04 2018
%Y Cf. A003592, A051664, A065119.
%K nonn
%O 1,1
%A Yuval Dekel (dekelyuval(AT)hotmail.com), Aug 02 2003
%E More terms from _T. D. Noe_, Feb 13 2012
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