OFFSET
1,3
COMMENTS
Square digits are 0, 1, 4 or 9.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Theta(n^(log_4 10))
MATHEMATICA
Select[FromDigits/@Tuples[{0, 1, 4, 9}, 4], !PrimeQ[#]&] (* Harvey P. Dale, Oct 14 2014 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Juri-Stepan Gerasimov, Mar 09 2009
EXTENSIONS
Formula from Charles R Greathouse IV, Mar 25 2010
STATUS
approved