The sequence $x_n$ with the recursion $x_1=1\ ,\ x_{n+1}=p_{x_n}$ , where $p_k$ denotes the $k-th$ prime, has the following values :
1 1
2 2
3 3
4 5
5 11
6 31
7 127
8 709
9 5 381
10 52 711
11 648 391
12 9 737 333
13 174 440 041
- How many values of this sequence are currently known ?