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

Offset: 1

Author

Yuval Dekel (dekelyuval(AT)hotmail.com), Aug 02 2003

Keywords

• nonn

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

Crossrefs

Cf. A003592, A051664, A065119.

Programs

Mathematica

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

PARI

is(n) = v = Vec(polcyclo(n)); sum(i=1,#v,v[i]!=0) == 5 \\ David A. Corneth, Jul 04 2018

Extensions

More terms from T. D. Noe, Feb 13 2012