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 A206347 #21 May 18 2025 18:02:32
%S A206347 21,27,33,60,117,153,222,228,306,426,480,495,558,585,615,636,669,684,
%T A206347 762,768,819,852,894,909,1083,1125,1131,1224,1239,1341,1455,1512,1539,
%U A206347 1776,1812,1845,2301,2484,2517,2541,2604,2706,2769,3093,3177
%N A206347 Numbers n such that 10*n+1, 20*n+1, and 30*n+1 are all primes.
%C A206347 (10*n+1)*(20*n+1)*(30*n+1) is a Carmichael number for all n in this sequence. Why is (6m+1)*(12m+1)*(18m +1) used to generate Carmichael numbers and never the formula (10m+1)*(20m+1)*(30m+1)?
%H A206347 Amiram Eldar, <a href="/A206347/b206347.txt">Table of n, a(n) for n = 1..10000</a>
%t A206347 Select[Range[20000], PrimeQ[10 #+1] && PrimeQ[20 #+1] && PrimeQ[30 #+1]&]
%t A206347 Select[Range[3500],AllTrue[1+{1,2,3}10#,PrimeQ]&] (* _Harvey P. Dale_, May 18 2025 *)
%Y A206347 Cf. A002997, A046025.
%K A206347 nonn
%O A206347 1,1
%A A206347 _José María Grau Ribas_, Feb 06 2012