A138921 Indices k such that A020508(k)=Phi[k](-9) is prime, where Phi is a cyclotomic polynomial.

3, 9, 12, 15, 20, 27, 33, 36, 59, 69, 91, 152, 207, 223, 232, 264, 336, 340, 380, 381, 492, 533, 540, 547, 549, 585, 615, 624, 627, 720, 773, 1009, 1287, 1320, 1428, 1537, 1823, 2093, 2401, 2724, 2733, 2804, 2920, 3316, 3803, 4124, 4132, 4620, 7143, 7520, 7708

Offset: 1

Author

M. F. Hasler, Apr 03 2008

Keywords

• nonn

Comments

Most terms of this sequence are multiples of 3, exceptions are 20, 59, 91, 152, 223, 232, 340, 380, 533, 547, 773... corresponding to a(n) with n=5, 9, 11, 12, 14, 15, 18, 19, 22, 24, 31...

Links

• Robert Price, Table of n, a(n) for n = 1..68
• Yves Gallot, Cyclotomic polynomials and prime numbers

Crossrefs

Cf. A020508, A138920-A138940.

Programs

Mathematica

Select[Range[1000], PrimeQ[Cyclotomic[#, -9]] &]

PARI

for( i=1,999, ispseudoprime( polcyclo(i,-9)) && print1( i",")) /* use ...subst(polcyclo(i),x,-9)... in PARI < 2.4.2 */

Extensions

a(32)-a(51) by Robert Price, Mar 22 2012