## Wednesday, November 30, 2016

### Checking if a number is prime: when to stop checking potential factors

Let's say you want to find out whether the number 888659800715261 is a prime number. If this were a programming exercise, a naive solution would be to go through all the numbers from 2 to 888659800715261/2 (half the number) and check whether any of the numbers divide 888659800715261 evenly. If none of them do, then the number is prime. The reason why we stop at half the number is because the largest factor that a number "n" can have before "n" itself is n/2, the one before that is n/3, and so on. Sure, we don't need to check every single number between 2 and n/2 as it would be enough to only check the prime numbers between 2 and n/2, but the point is that as soon as we reach n/2, we can stop checking and declare that 888659800715261 is prime.

The problem is that half of 888659800715261 is 444329900357630 which is still too big to check each number between 2 and it, probably even if you only check the prime numbers. The question is, is there an even lower upper-bound where we can stop checking potential factors in order to be sure that the number has no factors?

Let's look at which numbers between 1 and 24 are factors of 24:
    5  7  9 10 11  13 14 15 16 17 18 19 20 21 22 23 

In order for a number to be a factor of 24, there must be another factor which when the two factors are multiplied together give 24. For example, if 2 is a factor of 24, then there must be another factor of 24 which when multiplied by 2 gives 24. This other factor is 12. Let's call 2 and 12 co-factors. The co-factor of 3 is 8, the co-factor of 4 is 6. We can imagine co-factors being mirror reflections of each other:

    5  7  9 10 11  13 14 15 16 17 18 19 20 21 22 23 
(   [   {   <     >     }           ]                                     )


Notice how all the co-factors nest each other. The co-factors 4 and 6 are between the co-factors 3 and 8 which in turn are between the co-factors 2 and 12 which in turn are between 1 and 24. It seems like there is a line of reflection at 5, where all the factors smaller than 5 have co-factors larger than 5. This means that beyond 5, all factors are co-factors of smaller numbers.

Let's see another list of factors for 16:
  3  5 6 7  9 10 11 12 13 14 15 
(   [    { }        ]                       )


Notice how 4 is a co-factor with itself in this case, since 4 squared is 16. The line of reflection is crossed at 4 this time, which happens to be the square root of 16. In fact the square root of 24 is 4.898... which is close to 5 (the line of reflection of 24). This is always the case, because the line of reflection occurs where the two co-factors are the same. If the co-factors are not the same then one factor must be smaller than the square root whilst the other must be larger. If both are smaller or bigger then when multiplied together they will not give the number being factored. This is because if two co-factors "a" and "b" are supposed to equal "n", and both of them are larger than √n, then that would mean that a×b must also be larger than √n×√n, which means that a×b is larger than "n". The area of a rectangle with two large sides cannot be equal to the area of a rectangle with two small sides. Therefore, if the two sides are equal (a square) then in order to change the length of the sides without changing the area of the rectangle you must make one side larger whilst making the other smaller. This proves that the square root must be between any two co-factors.

So what's the point? The point is that if you reach the square root of "n" without having found any factors, then you are guaranteed that there will not be any factors beyond √n because if there are any then they must have co-factors smaller than √n, which you have already confirmed that there aren't any. So the square root of a number is the lower upper-bound to check for factors. Can there be an even lower upper-bound? No, because the number might be a square number and have no other factors apart from its square root, such as 25. Therefore the smallest co-factor a number can have is the square root of the number.

So in order to check if 888659800715261 is a prime number, we only have to check all the numbers up to 29810397 for factors, which is much better than 444329900357630. By the way, there is no number between 2 and 29810397 that evenly divides 888659800715261, which means that it is a prime number.