In the search for prime numbers among all Mersenne numbers to which can be primes or composites, an effective test for primality is known as the Lucas-Lehmer test. For an integer N the Lucas-Lehmer number is given by L(N)=L(N-1)-2. That is the square of the previous Lucas-Lehmer number minus 2, for examples, L(3)=14 where the previous LL-number is L(2)=4, 4=16 and 16-2=14 and it is divisible by the Mersenne prime 2-1=7, L(4)=194 where the previous LL-number is 14, L(5)=37634 where the previous LL-number is 194, and L(6)=1416317954 where the previous LL-number is 37634. Among these four LL-numbers, it can be demonstrated by simple calculations that even LL-numbers failed the test while the odd ones passed. Although this cannot be used to test all the consecutive primes, on the other hand, it can assert that all Mersenne numbers passing the test can be used to find all their associated perfect numbers. They are numbers that are equal to the sum of their positive divisors but not including the number itself, for examples: 6=1+2+3, 28=1+2+4+7+14, and 496=1+2+4+8+16+31+62+124+248. It can be noted that among these perfect numbers at the least two of their addends are primes. The largest prime to date that passes the Lucas-Lehmer test was discovered in the internet at http://www.mersenne.org/ see also the following links:
http://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality_test
http://mathworld.wolfram.com/Lucas-LehmerTest.html
http://primes.utm.edu/largest.html


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