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

1,1

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

Mathematica

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

Prog

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

Crossrefs

Cf. A020508, A138920-A138940.

Sequence in context: A077468 A089425 A255686 * A194412 A136290 A244147

Adjacent sequences: A138918 A138919 A138920 * A138922 A138923 A138924

Keyword

nonn

Author

M. F. Hasler, Apr 03 2008

Extensions

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

Status

approved