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 A138923 #18 Nov 11 2019 10:25:16 %S A138923 3,9,10,15,17,23,25,26,27,28,29,36,42,47,48,61,76,84,110,126,148,210, %T A138923 224,262,280,288,296,298,325,327,332,352,365,456,513,528,740,1062, %U A138923 1162,1445,1460,1518,1619,1660,2094,2130,2260,2380,3398,3447,3918 %N A138923 Indices k such that A020506(k)=Phi[k](-7) is prime, where Phi is a cyclotomic polynomial. %C A138923 There are only 6 terms between 365 and 1445 (exclusive). %H A138923 Robert Price, <a href="/A138923/b138923.txt">Table of n, a(n) for n = 1..97</a> %H A138923 Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a> %t A138923 Select[ Range[ 2400], PrimeQ[ Cyclotomic[#, -7]] &] (* _Robert G. Wilson v_, Mar 25 2012 *) %o A138923 (PARI) for( i=1,999, ispseudoprime( polcyclo(i,-7)) && print1( i",")) /* use ...subst(polcyclo(i),x,-7)... in PARI < 2.4.2 */ %Y A138923 Cf. A020506, A138920-A138940. %K A138923 nonn %O A138923 1,1 %A A138923 _M. F. Hasler_, Apr 03 2008