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 A124494 #19 Feb 02 2025 12:55:55
%S A124494 3,45,90,180,255,615,705,1350,1770,3225,5295,5775,5955,6060,8580,9855,
%T A124494 9945,11175,13095,13740,15195,21825,26820,26925,27615,28920,30075,
%U A124494 30705,31710,33375,35700,37350,37665,41250,43770,49185,50700,52185,53640
%N A124494 Numbers k for which 2*k-1, 4*k-1, 8*k-1 and 16*k-1 are primes.
%H A124494 Amiram Eldar, <a href="/A124494/b124494.txt">Table of n, a(n) for n = 1..10000</a>
%t A124494 Select[3*Range[20000], And @@ PrimeQ /@ ({2, 4, 8, 16}*# - 1) &] (* _Ray Chandler_, Nov 22 2006 *)
%t A124494 Select[Range[3,55000,3],AllTrue[2^Range[4] #-1,PrimeQ]&] (* or, faster *) Join[{3},Select[Range[ 15,55000,15],AllTrue[ 2^Range[4] #-1,PrimeQ]&]] (* _Harvey P. Dale_, Feb 02 2025 *)
%Y A124494 Cf. A006254, A124485, A125113, A124514, A124515, A124516.
%Y A124494 Subsequence of A124493. Supersequence of A124017.
%K A124494 nonn
%O A124494 1,1
%A A124494 _Artur Jasinski_, Nov 04 2006
%E A124494 Extended by _Ray Chandler_, Nov 22 2006