Quote:
Originally Posted by WhoCares
n= some 1523 digits number. I want to check 10^n+7 is prime or not.

There are in fact three alternatives: 10^n+7 is prime; 10^n+7 is not prime (composite); 10^n+7 is of unknown character.
You can find a factor for 10^n+7 by modular exponentiation but if not (which is fairly likely), then you will be stuck with the other two alternatives.
What is so special about that 10^n+7, though,  can you tell?