

A099080


Numbers n such that sigma(n).sigma(n1) ... sigma(2).sigma(1) is prime (dot between numbers means concatenation).


3




OFFSET

1,1


COMMENTS

Numbers of digits of primes corresponding to the four known terms of this sequence are respectively 2, 3, 133, and 232.
A naive heuristic suggests that this sequence is infinite but extremely sparse.  Charles R Greathouse IV, Nov 05 2013
There are no more terms below 10000.  Charles R Greathouse IV, Nov 09 2013


LINKS

Table of n, a(n) for n=1..4.
C. Rivera, Primes by Listing, The Prime Puzzles & Problems connection.
Eric Weisstein's World of Mathematics, Integer Sequence Primes


EXAMPLE

3 is in the sequence because sigma(3).sigma(2).sigma(1) = 431 is prime.


MATHEMATICA

Module[{nn=110, d}, d=DivisorSigma[1, Range[nn]]; Select[Range[nn], PrimeQ[ FromDigits[ Flatten[IntegerDigits/@Reverse[Take[d, #]]]]]&]] (* Harvey P. Dale, Jul 25 2016 *)


PROG

(PARI) s="1"; for(n=2, 1e3, s=Str(sigma(n), s); if(ispseudoprime(eval(s)), print1(n", "))) \\ Charles R Greathouse IV, Nov 05 2013


CROSSREFS

Cf. A046035, A099077, A099078, A099079.
Sequence in context: A356785 A015169 A041953 * A132532 A108023 A352163
Adjacent sequences: A099077 A099078 A099079 * A099081 A099082 A099083


KEYWORD

base,more,nonn


AUTHOR

Farideh Firoozbakht, Oct 23 2004


STATUS

approved



