## Wednesday, January 07, 2009

### Largest Prime Number

If you want to wile away an afternoon, try finding the largest prime number. For those that may have forgotten, a prime number is a number that is only evenly divisible by itself and 1. This definition eliminates all the even numbers that are evenly divisible by 2, leaving the odd numbers. The usual method of finding a prime number is to multiply 2 umpteen dozen times by itself and then subtracting 1 from that number, thereby creating an odd number (( 2 X 2 = 4), ( 4 - 1 = 3 )). You then take that odd number ( 3 ) and see if you can find a previous whole number up to 3, ( 1, 2, ) which will divide into it evenly leaving no remainder. There is a faster method. Numbers are written into a series of columns. Number 13 has 3 in column 0 and 1 in column 1. You will soon realize that any larger number can be easily created by adding another column For instance 3, 13, 213 etc.. It can also be seen that the total number of numbers is infinite because you just keep putting a number from 0 to 9 inclusive in the column to the left of a filled column.

Here are some rules:

A prime number, if it is a prime number , ends in 1, 3 , 7, 9 in column 0 ( far right column ). For instance 11, 13, 17, 19 are all prime numbers.

Any number whose digits, except for number 3, adds to 3 or an even multiple of 3 isn’t a prime number. For instance 39 isn’t a prime number because its’ digits ( 3 and 9 ) total 12 ( 3 + 9 = 12 ). ( 12/ 3 = 4 ).

A number, not a prime number, ending in 1, 3, 7, 9 in column 0 ( far right column ) is only evenly divisible by a number ending in 1, 3, 7, 9 in column 0 ( far right column ). For instance 39 is evenly divisible by 3 and 13 ( 3 X 13 = 39 ).