You can stop at sqrt($input).

Example:
The divisors of 100 are:
1, 2, 4, 5, 10, 20, 25, 50, 100. They come in pairs as follows:

1*100 = 100 
2*50 = 100
4* 25= 100
5*20 = 100
10*10 =100
20*5= 100

[download]
and so on. So at 10 which is sqrt(10) you are at the point where the second factor is smaller than the first and you have already tested for this pair.

In reply to Re: Re: YAPNC: Yet another prime number checker? by nefertari
in thread YAPNC: Yet another prime number checker? by snafu

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