trust me this is trivial... As a kid I had a teacher fond of asking if numbers were prime. Of course at the time I had no calculator and did not have many primes remembered. I did not even know the less than square root. I came up with a method that made a simple chart of smaller than the original number to use. I am wondering what this is called. Simple I would dived an odd number by 2. and take the higher and lower number. Say for 35 this would be 17, 18. Take 17: 17,16,15,14,13, and so on.. (17,1), start. (16,3), (15,5), (14,7), (13,11), (12,15) No need to go further. this provided me with a simple set with the odds to try and divide by, and only the factors of the original odd number are divisible such as 5, and 7 to whole numbers. A simple test was to subtract the number that was dividable from the start number, (17-15=2) and add it to the other paired number (18+2= 20) and of course divide it by the same number in this case 5. Note: this can be done from the number -1 down also. Faster from the bottom. With (34,1), (33,2) and so on. Of course this is just division by the odd numbers, yet I have not seen the use of the split. I did use it for factoring of other numbers also. It was fast. Any one know what this is called??