I observed one pattern in prime numbers which is applicable for numbers above 5, this is a shortcut to guess a higher number is a prime or not. For example if you are expecting a question in exam on prime number then this event increases the probability that number under observation is prime. So probability is 50-50. Second if number is not divisible by lower prime numbers like 2 3 5 7 11 few basic standard tests which we do then further it adds to that probability. Third if you have no time to go ahead with complex testing then try out following guess strategy, simply squar the number such as 31 square 961, 37 squar 1369, 41 squar 1681, 43 squar 1849, you can observe a pattern at ones and tens place of digit. Ones will always be 1 or 9 and tens will always be a even number. I tested this on few first hundred primes and this observation is consistent. Thus I think its a safe and smart way to guess a number is prime or not, but this is not foolproof, it just maximizes the chances that number is a prime by 90%.
Posted on: Fri, 23 Aug 2013 15:48:22 +0000
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