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 A381921 #15 Mar 12 2025 08:09:10
%S A381921 1,2,6,24,5040,40320,362880,
%T A381921 1982608315404440064116146708361898137544773690227268628106279599612729753600000000000000,
%U A381921 126886932185884164103433389335161480802865516174545192198801894375214704230400000000000000
%N A381921 Factorial numbers whose Hamming weight is also a factorial number.
%F A381921 a(n) = A000142(A381922(n)).
%e A381921 k = 7: A000120(A000142(7)) = 6, 6 is from A000142, thus 7! is a term.
%t A381921 With[{f = Range[100]!}, Select[f, MemberQ[f, DigitCount[#, 2, 1]] &]] (* _Amiram Eldar_, Mar 10 2025 *)
%Y A381921 Cf. A000120, A000142, A381922.
%K A381921 nonn,base
%O A381921 1,2
%A A381921 _Ctibor O. Zizka_, Mar 10 2025