OFFSET
1,1
EXAMPLE
a(1) = number of digits of 1996 = 4.
a(2) = number of digits of 19984 = 5.
a(3) = number of digits of 1999936 = 7 and
a(68)=7235, because smallest prime of the form 5^(2*68-1)*10^m -1 is 5^135*10^7099-1 and 4^68(5^135*10^7099-1) has 7235 digits.
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Farideh Firoozbakht, Jan 06 2015
STATUS
approved