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 A137827 #18 Feb 16 2025 08:33:07 %S A137827 4,7,13,16,19,25,31,37,43,49,61,64,67,73,79,97,103,109,121,127,139, %T A137827 151,157,163,169,181,193,199,211,223,229,241,256,271,277,283,289,307, %U A137827 313,331,337,343,349,361,367,373,379,397,409,421,433,439,457,463,487,499 %N A137827 Prime powers (A246655) congruent to 1 (mod 3). %C A137827 Numbers k not being powers of 3 such that x^2 + x + 1 (or x^2 - x + 1) is reducible over GF(k). - _Jianing Song_, Sep 24 2019 %H A137827 Antti Karttunen, <a href="/A137827/b137827.txt">Table of n, a(n) for n = 1..10000</a> %H A137827 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CyclotomicGraph.html">Cyclotomic Graph</a> %t A137827 Select[ Range[4, 500], (Mod[#, 3] == 1 && Mod[#, # - EulerPhi[#]] == 0)& ] (* _Jean-François Alcover_, Oct 26 2012 *) %o A137827 (PARI) isok(n) = isprimepower(n) && ((n % 3) == 1); \\ _Michel Marcus_, Sep 24 2019 %Y A137827 Row 3 of A354940 (conjectured). %Y A137827 Intersection of A016777 and A246655. %Y A137827 Cf. A354982 (characteristic function), A354984 (terms * 3). %Y A137827 Cf. also A327752, A327753. %K A137827 nonn,easy %O A137827 1,1 %A A137827 _Eric W. Weisstein_, Feb 12 2008