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 A332242 #19 Feb 09 2020 03:01:17
%S A332242 1,48,49,53,54,57
%N A332242 Numbers k such that k! has exactly k nonzero decimal digits.
%C A332242 No other terms < 100000.
%C A332242 Conjecture: these 6 terms are the only terms of the sequence, i.e., there are no terms larger than 57.
%F A332242 { k : A034886(k) - A027869(k) = k }.
%e A332242 48! = 12413915592536072670862289047373375038521486354677760000000000 has 48 nonzero decimal digits, so 48 is a term.
%p A332242 q:= n-> nops(subs(0=NULL, convert(n!, base, 10)))=n:
%p A332242 select(q, [$0..100])[]; # _Alois P. Heinz_, Feb 07 2020
%o A332242 (Python)
%o A332242 A332242_list, i, n = [], 0, 1
%o A332242 while i < 1000:
%o A332242 s = str(n)
%o A332242 if len(s) - s.count('0') == i:
%o A332242 A332242_list.append(i)
%o A332242 i += 1
%o A332242 n *= i
%o A332242 (PARI) isok(k) = #select(x->(x != 0), digits(k!)) == k; \\ _Michel Marcus_, Feb 08 2020
%Y A332242 Cf. A000142, A027869, A034886.
%K A332242 nonn,base
%O A332242 1,2
%A A332242 _Chai Wah Wu_, Feb 07 2020