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 A349424 #30 Mar 04 2023 14:48:31
%S A349424 1,2,4,6,12,24,36,48,120,144,240,576,720,1440,2880,4320,5040,10080,
%T A349424 14400,17280,30240,40320,80640,86400,120960,241920,362880,518400,
%U A349424 604800,725760,967680,2177280,3628800,4838400,7257600,8709120,21772800,25401600,29030400
%N A349424 Integers that are the product of two factorials A000142.
%C A349424 The first term a(1) = 0!*0! = 1!*0! = 1!*1!.
%C A349424 This is also the number of perfect matchings in a graph connecting C-G and A-U pairs in a RNA string (see Rosalind link). - _Lorenzo Pieri_, Mar 02 2023
%H A349424 Rosalind, <a href="https://rosalind.info/problems/pmch/">Perfect Matchings and RNA Secondary Structures</a>.
%e A349424 720 is a term since 720 = 5!*3! = 6!*0! = 6!*1!.
%t A349424 (Union@Flatten@Table[x!*y!, {x, 0, 100}, {y, 0, x}])[[1 ;; 50]]
%o A349424 (PARI) lista(nn) = select(x->(x<2*nn!), setbinop((x,y)->x!*y!, [1..nn])); \\ _Michel Marcus_, Nov 17 2021
%Y A349424 Cf. A000142, A098361.
%Y A349424 Subsequence of A001013.
%K A349424 nonn
%O A349424 1,2
%A A349424 _Frank M Jackson_, Nov 17 2021