Project Euler Problem 7 finding the 10001st prime

As previously mentioned in my post before I’m not going to post the complete source code for my solutions (written in C#). Instead I’m just going to talk through some of the techniques that I’ve used to solve the problems.

Here is problem 7:
By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10001st prime number?

Now this is actually quite easy (and quick) to solve if you know a little trick with primes. As you probably know a number is said to be prime if it’s divisible by itself and 1. A quick way of checking to see if it’s prime is to ensure that it’s not divisble by any prime lower than the square root of the number that you are checking.

Obviously as you find a new prime number you need to add it to your running list of primes so that can be used to check subsequent numbers. Using this technique gives you the answer very fast.

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