Pg(k) primes with k prime
 

Pg(k) numbers=(2^k-1)*10^d+2^(k-1)-1 where d is the number of decimal digits of 2^(k-1)-1.


For k>2, these are the values of k such that both Pg(k) and k are prime:


3,7,19,67,79,359,2131,3371,331259


These primes are all of the form 4*s+3, with s>=0.


Can it be proven that if Pg(k) is prime and k is prime, then k must have the form 4*s+3?

Proof
 

Ok it can be easily proven...never mind!