I have worked on these at the same time I worked on 2*a^a+-1. N must

be prime in order for the above to be prime. I have searched for

PRP's for a large n value (above 10000) and found only these 4.

Probably the number of primes of this type are finite.

As for n=13 and 19, both are prime.

Harsh Aggarwal

--- In

primenumbers@yahoogroups.com, "julienbenney" <jpbenney@f...>

wrote:

> Moving away from prime quadruples, there is another topic that I

have

> never seen anything written about that I am curious about.

>

> I know from tables of generalised repunit primes that ((n^n)-1)/(n-

1)

> is prime for n = 2, 3, 19 and 31 and for no other n up to 1,000. Is

> there a theorem that could show these four numbers to be the only n

for

> which ((n^n)-1)/(n-1) is prime?

>

> Also, when was the primality of 19^19-1/18 and 31^31-1/30 proved?

[I am

> curious]