This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A138920 #26 Apr 03 2023 10:36:11 %S A138920 4,5,7,12,19,24,31,36,38,46,48,53,67,75,78,120,186,196,293,320,327, %T A138920 369,634,641,713,770,931,1067,1172,1194,1404,1452,1752,1812,1836,1844, %U A138920 1875,1890,2062,2137,2177,2232,2264,3011,3042,3261,3341,4775,5334,6685 %N A138920 Indices k such that A020509(k)=Phi[k](-10) is prime, where Phi is a cyclotomic polynomial. %C A138920 "Unique [period] primes" (A040017) are often of the form Phi[k](10) or Phi[k](-10). %C A138920 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 %H A138920 Ray Chandler, <a href="/A138920/b138920.txt">Table of n, a(n) for n = 1..89</a> (first 76 terms from Robert Price) %H A138920 C. Caldwell, <a href="https://t5k.org/glossary/page.php?sort=UniquePrime">Unique Primes</a>. %H A138920 <a href="/index/Cy#CyclotomicPolynomialsValuesAtX">Index entries for cyclotomic polynomials, values at X</a> %t A138920 Select[Range[1000], PrimeQ[Cyclotomic[#, -10]] &] %o A138920 (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)... */ %Y A138920 Cf. A019328, A040017, A138940. %K A138920 nonn %O A138920 1,1 %A A138920 _M. F. Hasler_, Apr 03 2008 %E A138920 a(28)-a(43) from _Robert Price_, Mar 09 2012 %E A138920 a(44)-a(50) from _Robert Price_, Apr 14 2012