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A138921
Indices k such that A020508(k)=Phi[k](-9) is prime, where Phi is a cyclotomic polynomial.
2
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...
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
KEYWORD
nonn
AUTHOR
M. F. Hasler, Apr 03 2008
EXTENSIONS
a(32)-a(51) by Robert Price, Mar 22 2012
STATUS
approved