OFFSET
1,2
COMMENTS
sigma(x) is even unless x is a square or twice a square, therefore 10^n+sigma(n) can't be prime unless n is a square or twice a square, and {1, 2, 4, 242} are the only solutions < 5000.
a(25) > 10^5. - Robert Price, May 08 2015
MATHEMATICA
Select[Range[0, 10000], PrimeQ[10^# + DivisorSigma[1, #^2]] &] (*
Robert Price, May 08 2015 *)
PROG
(PARI) for(n=1, 9999, ispseudoprime(10^n+sigma(n^2)) && print1(n", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Aug 07 2011
EXTENSIONS
a(19)-a(24) from Robert Price, May 08 2015
STATUS
approved