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Old 2017-04-17, 15:07   #10
Batalov's Avatar
Mar 2008

217038 Posts

Originally Posted by WhoCares View Post
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?
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