

A193881


Numbers n such that 10^nsigma(n^2) is prime.


2




OFFSET

1,1


COMMENTS

sigma(x) is even unless x is a square or twice a square, therefore 10^nsigma(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^nsigma(n^2)) && print1(n", "))
(Magma) [n: n in [1..450]  IsPrime(10^nDivisorSigma(1, n^2))]; // Vincenzo Librandi, Mar 26 2015


CROSSREFS

Cf. A110065, A110066, A110067, A173837, A174176.
Sequence in context: A349532 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



