## Saturday, April 23, 2005

### Some New Ways To Choose A Probable Prime

A prime number is a number that can only be divided evenly by itself and 1. The only way to definitely calculate whether a prime is a prime is by dividing it by all the numbers that preceded the number you suspect is a prime. There are, however, ways of choosing the number that you may suspect is a prime before dividing. First of all any number that has more than 1 digit isn’t a prime if it can be divided by 2 and 5. For instance 12 and 15 aren’t primes because 12 / 2 = 6 and 15 / 5 = 3. Secondly, most, if not all primes, end with the last digit being 1, 3, 7, and 9. For instance 11, 13, 17, and 19 are primes. Take any chosen number such as 11 for instance and break it down into its’ constituents remembering that for a potential prime number the number has to end in 1, 3, 7, 9. Therefore 11, for example, can be broken down into (10 +1), (2+9), (4+7), (8+3) . Remove the (+)’s. The numbers are 101, 29, 47 ,83. Test the numbers. You will find in this case that 101, 29, 47, and 83 are all prime numbers. Sometimes these numbers aren’t prime. The difference between a prime number and the next prime number is some multiple of 2. For instance the difference between 137 and 139 is 2. Between 151 and 157 is 6 (2 X 3). Between 181 and 191 is 10 (2X5).