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 A250177 #15 Apr 15 2018 05:07:49
%S A250177 3,6,7,12,22,27,28,35,41,59,63,69,112,127,132,133,136,140,164,166,202,
%T A250177 215,218,276,288,307,323,334,343,377,383,433,474,479,516,519,521,532,
%U A250177 538,549,575,586,622,647,675,680,692,733,790,815,822,902,909,911,915,952,966,1025,1034,1048,1093
%N A250177 Numbers n such that Phi_21(n) is prime, where Phi is the cyclotomic polynomial.
%H A250177 G. C. Greubel, <a href="/A250177/b250177.txt">Table of n, a(n) for n = 1..10000</a>
%t A250177 a250177[n_] := Select[Range[n], PrimeQ@Cyclotomic[21, #] &]; a250177[1100] (* _Michael De Vlieger_, Dec 25 2014 *)
%o A250177 (PARI) {is(n)=isprime(polcyclo(21,n))};
%o A250177 for(n=1,100, if(is(n)==1, print1(n, ", "), 0)) \\ _G. C. Greubel_, Apr 14 2018
%Y A250177 Cf. A008864 (1), A006093 (2), A002384 (3), A005574 (4), A049409 (5), A055494 (6), A100330 (7), A000068 (8), A153439 (9), A250392 (10), A162862 (11), A246397 (12), A217070 (13), A250174 (14), A250175 (15), A006314 (16), A217071 (17), A164989 (18), A217072 (19), A250176 (20), this sequence (21), A250178 (22), A217073 (23), A250179 (24), A250180 (25), A250181 (26), A153440 (27), A250182 (28), A217074 (29), A250183 (30), A217075 (31), A006313 (32), A250184 (33), A250185 (34), A250186 (35), A097475 (36), A217076 (37), A250187 (38), A250188 (39), A250189 (40), A217077 (41), A250190 (42), A217078 (43), A250191 (44), A250192 (45), A250193 (46), A217079 (47), A250194 (48), A250195 (49), A250196 (50), A217080 (53), A217081 (59), A217082 (61), A006315 (64), A217083 (67), A217084 (71), A217085 (73), A217086 (79), A153441 (81), A217087 (83), A217088 (89), A217089 (97), A006316 (128), A153442 (243), A056994 (256), A056995 (512), A057465 (1024), A057002 (2048), A088361 (4096), A088362 (8192), A226528 (16384), A226529 (32768), A226530 (65536), A251597 (131072), A244150 (524287), A243959 (1048576).
%Y A250177 Cf. A085398 (Least k>1 such that Phi_n(k) is prime).
%K A250177 nonn
%O A250177 1,1
%A A250177 _Eric Chen_, Dec 24 2014