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Indices k such that A020507(k)=Phi[k](-8) is prime, where Phi is a cyclotomic polynomial.
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%I #12 Nov 11 2019 13:47:38

%S 6,15,18,21,39,69,159,165,174,231,378,408,501,504,606,873,897,1122,

%T 1209,1395,1947,2040,2361,2778,3369,7305,7647,8154,8331,9483,13071,

%U 14616,15819,20301,21282,27144,31083,34725,35775,36855,38118,39360

%N Indices k such that A020507(k)=Phi[k](-8) is prime, where Phi is a cyclotomic polynomial.

%H Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a>

%t Select[Range[1000], PrimeQ[Cyclotomic[#, -8]] &]

%o (PARI) for( i=1,999, ispseudoprime( polcyclo(i,-8)) && print1( i",")) /* use ...subst(polcyclo(i),x,-8)... in PARI < 2.4.2 */

%Y Cf. A020507, A138920-A138940.

%K nonn,more

%O 1,1

%A _M. F. Hasler_, Apr 03 2008

%E a(18)-a(42) from _Robert Price_, Mar 25 2012