A193882 Numbers n such that 10^n+sigma(n^2) is prime.

1, 2, 3, 8, 13, 15, 19, 20, 41, 47, 50, 76, 100, 162, 204, 310, 318, 536, 2502, 4016, 5612, 5849, 52753, 64843

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

Links

Table of n, a(n) for n=1..24.

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

Cf. A110065, A110066, A110067, A173837, A174176, A193881.

Sequence in context: A086590 A160610 A127484 * A168343 A217330 A358207

Adjacent sequences: A193879 A193880 A193881 * A193883 A193884 A193885

Keyword

nonn

Author

M. F. Hasler, Aug 07 2011

Extensions

a(19)-a(24) from Robert Price, May 08 2015

Status

approved