Numbers With 3 Divisors

August 30, 2019

I saw this question on Stack Overflow a few days ago; it’s a well-known problem in number theory:

For a given number n, how many numbers less than n have exactly 3 divisors?

Your task is to write a program that counts the numbers less than n that have exactly 3 divisors, and use it to find the result when n is one billion. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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