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A086761

Numbers k such that k-th cyclotomic polynomial has exactly 5 nonzero terms.

5, 10, 20, 25, 40, 50, 80, 100, 125, 160, 200, 250, 320, 400, 500, 625, 640, 800, 1000, 1250, 1280, 1600, 2000, 2500, 2560, 3125, 3200, 4000, 5000, 5120, 6250, 6400, 8000, 10000, 10240, 12500, 12800, 15625, 16000, 20000, 20480, 25000, 25600, 31250, 32000

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OFFSET

1,1

COMMENTS

A206787(a(n)) = 6. - Reinhard Zumkeller, Feb 12 2012

All terms have the form 2^a 5^b with a >= 0 and b > 0. - T. D. Noe, Feb 13 2012

If the above holds for all terms then this sequence is 5 * A003592. - David A. Corneth, Jul 04 2018

LINKS

Table of n, a(n) for n=1..45.

MATHEMATICA

Select[Range[1000], Count[CoefficientList[Cyclotomic[#, x], x], 0] == EulerPhi[#] - 4 &] (* T. D. Noe, Feb 13 2012 *)

PROG