Concatenation problem

[Maximilian Hasler]

>     a(1) = n
>     a(n+1) = k where concat(a(i),k) and concat(k,a(i)) are both prime for
>  all 1 <= i <= k.
>
>  In other words, the elements are chosen greedily so that the concatenation
>  of any two distinct elements is prime. The elements themselves are not
>  necessarily prime.

you can put a link to
http://projecteuler.net/index.php?section=problems&id=60

obvious observations :
If 2 is in the sequence, there are no other terms.
The first element not divisible by 3 determines the residue mod 3 of
all subsequent terms not divisible by 3
(since if there is one element = 1 mod 3, there cannot be another = 2
mod 3, and conversely.
Maximilian