None of them are at all feasible when trying to determine the primeness of a 200 digit number. The square root of a 200 digit number is a 100 digit number. Even if the first 200 digit number you are going to try is prime, and take your last suggestion, you'd need to keep track of all the prime numbers less than 100 digits long.

Considering there are about n / ln n prime numbers less than n, you'd need to keep track of about 4 * 10**98 prime numbers.

Many people in this thread just don't seem to realize how big a 200 digit number is.

Abigail

In reply to Re: Finding Primes by Abigail-II
in thread Finding Primes by Tommy

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