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%I #15 Nov 11 2019 10:25:09
%S 3,9,12,15,20,27,33,36,59,69,91,152,207,223,232,264,336,340,380,381,
%T 492,533,540,547,549,585,615,624,627,720,773,1009,1287,1320,1428,1537,
%U 1823,2093,2401,2724,2733,2804,2920,3316,3803,4124,4132,4620,7143,7520,7708
%N Indices k such that A020508(k)=Phi[k](-9) is prime, where Phi is a cyclotomic polynomial.
%C 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...
%H Robert Price, <a href="/A138921/b138921.txt">Table of n, a(n) for n = 1..68</a>
%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[#, -9]] &]
%o (PARI) for( i=1,999, ispseudoprime( polcyclo(i,-9)) && print1( i",")) /* use ...subst(polcyclo(i),x,-9)... in PARI < 2.4.2 */
%Y Cf. A020508, A138920-A138940.
%K nonn
%O 1,1
%A _M. F. Hasler_, Apr 03 2008
%E a(32)-a(51) by Robert Price, Mar 22 2012