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 A138920 Indices k such that A020509(k)=Phi[k](-10) is prime, where Phi is a cyclotomic polynomial. 20
 4, 5, 7, 12, 19, 24, 31, 36, 38, 46, 48, 53, 67, 75, 78, 120, 186, 196, 293, 320, 327, 369, 634, 641, 713, 770, 931, 1067, 1172, 1194, 1404, 1452, 1752, 1812, 1836, 1844, 1875, 1890, 2062, 2137, 2177, 2232, 2264, 3011, 3042, 3261, 3341, 4775, 5334, 6685 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS "Unique [period] primes" (A040017) are often of the form Phi[k](10) or Phi[k](-10). Two cyclotomic polynomial identities tightly connect this sequence to A138940: 1) Phi_2k(x) = Phi_k(-x) for odd integer k > 1. 2) Phi_4k(x) = Phi_2k(x^2) for all positive integer k. - Ray Chandler, Apr 30 2017 LINKS Ray Chandler, Table of n, a(n) for n = 1..89 (first 76 terms from Robert Price) C. Caldwell, Unique Primes. MATHEMATICA Select[Range[1000], PrimeQ[Cyclotomic[#, -10]] &] PROG (PARI) for( i=1, 999, is/*pseudo*/prime( polcyclo(i, -10)) &&& print1( i", ")) /* for PARI < 2.4.2 use ...subst(polcyclo(i, x), x, -10)... */ CROSSREFS Cf. A019328, A040017, A138940. Sequence in context: A160934 A032390 A112918 * A309833 A048224 A064237 Adjacent sequences:  A138917 A138918 A138919 * A138921 A138922 A138923 KEYWORD nonn AUTHOR M. F. Hasler, Apr 03 2008 EXTENSIONS a(28)-a(43) from Robert Price, Mar 09 2012 a(44)-a(50) from Robert Price, Apr 14 2012 STATUS approved

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Last modified June 1 09:53 EDT 2020. Contains 334762 sequences. (Running on oeis4.)