# prime factoring stupid question..

by lostcauses10x
Tags: factoring, prime, stupid
 P: 94 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??
 P: 2,427 solution by trial and error? Normally prime numbers are tested using trial division: http://en.wikipedia.org/wiki/Prime_number
 P: 234 Usually you can also do test by divisibility for small prime numbers, whether a number is divisible by 2, 3, 5, 7, 11. The goal is not to divide them but to test whether they can be divided. Trivial example without knowing 9x3=27. 27? 2+7=9 9/3 = 3, so 27 is divisible by 3. http://en.wikipedia.org/wiki/Divisibility_rule Other than that if you can remember 1-10 or even further multiplication table, you will know what numbers are divisible by what rather quickly without doing computation. This is what most students in my school are taught with.
P: 94

## prime factoring stupid question..

Well this was just some thing a small child learned a long time ago. 40 years +..

removed.

It uses a number less than half the original.

Thanks for looking, was just wondering if such had a name.

edited due to this "This routine consists of dividing n by each integer m that is greater than 1 and less than or equal to the square root of n." From the above link.
It would fall under a methodical up the line from 1 of the evens.
Yet I have not found reference to splitting the number to do so..
Math
Emeritus