Ans: 5 .
Solution: Consider the numbers a1= 4p^2+1 and a2=6p^2+1 modulo 5.
It is easy to show that one of them is always 0 mod 5 for any k if the prime is 5n+k, and k is not 0.
The only prime number with k=0 is 5.
Then a1=101 and a2=151, both primes.
Edited on September 27, 2016, 8:26 am