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Efficient prime test

, 12 Nov 2013 CPOL
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The IsPrime algorithm will always find a number that divides n if it is composed.

Introduction

Prime numbers are essential in many fields of mathematics and computer science, especially cryptography. An interesting problem arises when we need to decide whether an integer is a prime number.

The straightforward, naïve algorithm for deciding if a number n is prime follows a procedure in which a loop from 2 to n-1 checks every time whether the number representing the step of the loop divides n and in that case returns false.

The previous algorithm runs in O (n). An improvement to the running time of the Naïve prime test can be achieved if we ask ourselves the question: Is it really necessary to loop from 2 to n-1? The answer to this question is no, is not necessary, it would be enough to loop only from 2 to squart(n).

Correctness of the IsPrime algorithm: Let’s assume that all numbers that divide n are greater than squart(n). If this is the case then the smallest number that can divide n is squart(n)+1, but, if n is composed then the smallest numbers dividing n must be greater or equal than [squart(n)+1] [squart(n)+1], but this product is greater than n, which is a contradiction, though there must be at least a number dividing n less than or equal to squart(n) proving that the IsPrime algorithm will always find a number that divides n if it is composed.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

arnaldo.skywalker
Software Developer
Cuba Cuba
I'm a programmer and mathematician, graduated of Computer Science. Lover of Jazz, music, cinema and art in general.

Comments and Discussions

 
QuestionThis is not "efficient" prime test Pinmemberfulloflove28-Feb-14 17:15 
QuestionNot the prime example of optimization PinmemberCaldasGSM12-Nov-13 7:37 
AnswerRe: Not the prime example of optimization Pinmemberarnaldo.skywalker12-Nov-13 8:36 
GeneralMessage Removed Pinmemberbharat_h0312-Nov-13 22:49 
GeneralRe: Not the prime example of optimization Pinmemberarnaldo.skywalker14-Nov-13 4:07 
GeneralRe: Not the prime example of optimization Pinmemberbharat_h0321-Nov-13 0:01 
i was wrong. :(
QuestionThere are several prime sieves. PinprotectorPete O'Hanlon12-Nov-13 5:59 
AnswerRe: There are several prime sieves. Pinmemberarnaldo.skywalker12-Nov-13 6:53 
GeneralRe: There are several prime sieves. PinprofessionalPIEBALDconsult12-Nov-13 11:06 
GeneralRe: There are several prime sieves. Pinmemberarnaldo.skywalker14-Nov-13 4:11 

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