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