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 A138921 #15 Nov 11 2019 10:25:09 %S A138921 3,9,12,15,20,27,33,36,59,69,91,152,207,223,232,264,336,340,380,381, %T A138921 492,533,540,547,549,585,615,624,627,720,773,1009,1287,1320,1428,1537, %U A138921 1823,2093,2401,2724,2733,2804,2920,3316,3803,4124,4132,4620,7143,7520,7708 %N A138921 Indices k such that A020508(k)=Phi[k](-9) is prime, where Phi is a cyclotomic polynomial. %C A138921 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 A138921 Robert Price, <a href="/A138921/b138921.txt">Table of n, a(n) for n = 1..68</a> %H A138921 Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a> %t A138921 Select[Range[1000], PrimeQ[Cyclotomic[#, -9]] &] %o A138921 (PARI) for( i=1,999, ispseudoprime( polcyclo(i,-9)) && print1( i",")) /* use ...subst(polcyclo(i),x,-9)... in PARI < 2.4.2 */ %Y A138921 Cf. A020508, A138920-A138940. %K A138921 nonn %O A138921 1,1 %A A138921 _M. F. Hasler_, Apr 03 2008 %E A138921 a(32)-a(51) by Robert Price, Mar 22 2012