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 A138928 #20 Nov 11 2019 10:25:58 %S A138928 3,4,5,6,7,8,10,11,12,13,14,15,16,17,18,19,21,23,24,26,30,31,32,34,38, %T A138928 39,40,43,45,49,54,56,61,62,63,66,75,79,80,85,87,98,101,117,120,122, %U A138928 127,130,138,154,161,167,170,178,183,184,186,187,191,192,199,205,207,208 %N A138928 Indices n such that A020501(n) = phi(n)(-2) is prime, where Phi is a cyclotomic polynomial. %C A138928 While the sequence is not very interesting up to a(n)<300, there are only 4 values in the interval [400,599]. %H A138928 Robert Price, <a href="/A138928/b138928.txt">Table of n, a(n) for n = 1..227</a> %H A138928 Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a> %H A138928 <a href="/index/Cy#CyclotomicPolynomialsValuesAtX">Index entries for cyclotomic polynomials, values at X</a> %t A138928 Select[ Range[3, 1000], PrimeQ[ Cyclotomic[#, -2]] &] (* _Robert G. Wilson v_, Mar 25 2012 *) %o A138928 (PARI) for( i=1,999, isprime( polcyclo(i,-2)) && print1( i",")) /* for PARI < 2.4.2 use ...subst(polcyclo(i),x,-2)...*/ %Y A138928 Cf. A020501, A138920-A138940. %K A138928 nonn %O A138928 1,1 %A A138928 _M. F. Hasler_, Apr 03 2008