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 A144856 #23 Apr 03 2023 10:36:11
%S A144856 21,93,217,381,889,3937,24573,57337,253921,393213,917497,1040257,
%T A144856 1572861,3670009,4063201,16252897,16646017,66584449,1073602561,
%U A144856 4294434817,6442450941,15032385529,66571993057,68718821377,272730423169
%N A144856 Semiprimes that are a product of distinct Mersenne primes.
%C A144856 Since each Mersenne prime is congruent to -1 (mod 4), it is easy to see that a(n) == 1 (mod 4). - _Timothy L. Tiffin_, Jul 07 2021
%H A144856 T. D. Noe, <a href="/A144856/b144856.txt">Table of n, a(n) for n = 1..90</a>
%H A144856 G. L. Honaker, Jr. and Chris Caldwell, <a href="https://t5k.org/curios/page.php?curio_id=17197">Prime Curios! 21</a>
%H A144856 Googology Wiki,<a href="https://googology.wikia.org/wiki/Largest_known_squarefree_semiprime">Largest known squarefree semiprime</a>
%e A144856 a(1) = 3*7 = 21, a(2) = 3*31 = 93, a(3) = 7*31 = 217, ... - _Timothy L. Tiffin_, Jul 07 2021
%t A144856 Mp = 2^{2, 3, 5, 7, 13, 17, 19, 31, 61} - 1; Take[ Union[ Times @@@ Subsets[ Mp, {2}]], 25] (* _Robert G. Wilson v_, Sep 25 2008 *)
%Y A144856 Cf. A000668, A001358. Subsequence of A144482 (semiprimes that are the product of Mersenne primes).
%K A144856 nonn
%O A144856 1,1
%A A144856 _G. L. Honaker, Jr._, Sep 22 2008
%E A144856 More terms from _Robert G. Wilson v_, Sep 25 2008
%E A144856 More terms from _T. D. Noe_, Sep 25 2008