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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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified August 17 12:30 EDT 2022. Contains 356189 sequences. (Running on oeis4.)