OFFSET
1,1
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 {2, 49} are the only solutions < 10^4.
a(10) > 10^5. - Robert Price, Mar 24 2015
MATHEMATICA
Select[Range[1000], PrimeQ[10^# - DivisorSigma[1, #^2]] &] (* Robert Price, Mar 24 2015 *)
PROG
(PARI) for(n=1, 9999, ispseudoprime(t=10^n-sigma(n^2)) && print1(n", "))
(Magma) [n: n in [1..450] | IsPrime(10^n-DivisorSigma(1, n^2))]; // Vincenzo Librandi, Mar 26 2015
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
M. F. Hasler, Aug 07 2011
EXTENSIONS
a(6)-a(9) from Robert Price, Mar 25 2015
STATUS
approved