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 A035065 #24 Sep 08 2022 08:44:52 %S A035065 4,5,6,8,10,14,15,20,23,27,29,33,35,39,43,51,58,68,70,84,86,89,90,95, %T A035065 104,107,110,111,116,117,119,120,133,134,136,139,147,150,158,159,170, %U A035065 183,193,199,206,211,224,229,235,239,244,249,254,270,279,282,291,299 %N A035065 Numbers k such that k! has a prime number of digits. %H A035065 Robert Israel, <a href="/A035065/b035065.txt">Table of n, a(n) for n = 1..10000</a> %H A035065 <a href="/index/Fa#factorial">Index entries for sequences related to factorial numbers</a> %e A035065 a(1)=4 because 4! = 24 has 2 (a prime) digits. %e A035065 23! = 25852016738884976640000 has exactly 23 digits! %p A035065 filter:= n -> isprime(1+ilog10(n!)): %p A035065 select(filter, [$1..1000]); # _Robert Israel_, Mar 27 2018 %t A035065 Select[ Range[300], PrimeQ[ Floor[ Log[10, #! ] + 1]] & ] %o A035065 (PARI) isok(n) = isprime(#digits(n!)); \\ _Michel Marcus_, Mar 28 2018 %o A035065 (Magma) [n: n in [1..300] | IsPrime(Floor(Log(10, Factorial(n))+1))]; // _Vincenzo Librandi_, Mar 28 2018 %Y A035065 Cf. A000142, A034886, A035066, A067367. %K A035065 nonn,base %O A035065 1,1 %A A035065 _Patrick De Geest_, Nov 15 1998 %E A035065 Offset corrected by _Robert Israel_, Mar 27 2018