OFFSET
1,1
LINKS
EXAMPLE
a(1)=4 because 4! = 24 has 2 (a prime) digits.
23! = 25852016738884976640000 has exactly 23 digits!
MAPLE
filter:= n -> isprime(1+ilog10(n!)):
select(filter, [$1..1000]); # Robert Israel, Mar 27 2018
MATHEMATICA
Select[ Range[300], PrimeQ[ Floor[ Log[10, #! ] + 1]] & ]
PROG
(PARI) isok(n) = isprime(#digits(n!)); \\ Michel Marcus, Mar 28 2018
(Magma) [n: n in [1..300] | IsPrime(Floor(Log(10, Factorial(n))+1))]; // Vincenzo Librandi, Mar 28 2018
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Nov 15 1998
EXTENSIONS
Offset corrected by Robert Israel, Mar 27 2018
STATUS
approved