A193881 Numbers n such that 10^n-sigma(n^2) is prime.

7, 52, 446, 614, 1137, 4852, 5615, 22154, 71291

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

Links

Table of n, a(n) for n=1..9.

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

Cf. A110065, A110066, A110067, A173837, A174176.

Sequence in context: A369147 A162233 A185623 * A249349 A300492 A179517

Adjacent sequences: A193878 A193879 A193880 * A193882 A193883 A193884

Keyword

nonn,hard

Author

M. F. Hasler, Aug 07 2011

Extensions

a(6)-a(9) from Robert Price, Mar 25 2015

Status

approved