[AI] formula for prime numbers

Amiyo Biswas amiyo.biswas at gmail.com
Thu Sep 2 02:39:26 EDT 2010


Hello,

There are some similar tricks for determining if a number is a prime one. If 
you add the last two digits and the previous ones and the result is 
divisible by 11, the number is divisible by 11. In this case of Kartik's 
example, 21 +23 = 44 which is divisible by 11. I have forgotten some tricks 
which can determine dibisibility by 7.

In C / C++ lessons, you may be asked to write a utility for finding out a 
prime number.

With best regards,
Amiyo Biswas.
Cell: 9433464329
----- Original Message ----- 
From: "Kartik Sawhney" <sawhney.kartik at gmail.com>
To: <accessindia at accessindia.org.in>
Sent: Thursday, September 02, 2010 11:01 AM
Subject: Re: [AI] formula for prime numbers


> Hi,
>
> In order to compute and determine if a large number is
> prime/composite, adopt the following method:
>
> For example sake I'll use the number 2,321
>
> 1) Find the square root of the number - The Square root of 2,321 is 
> 48.176...
>
> 2) remember that if the square root results in an integer it is
> automatically composite
>
> 3) if your number ends in a 0,2,4,5,6,8 it is NOT PRIME
> - 2,321 ends in a 1, so we keep going
>
> 4) add up the digits of your number, if the sum is divisible by 3,
> your number is composite (basically testing the divisibility of the
> number by 3)-2+3+2+1= 8 3 does not go into 8 evenly- onto the next
> step
>
> 5) Divide the number by all the prime numbers less than the square
> root (you can skip 2, 3, and 5)
> -since the square root of 2,321 is 48.176..., we need to
> try dividing 2,321 by primes less than 48
> (7,11,13,17,19,23,29,31,37,41,43,47) since 2,321 is
> divisible by 11, it is NOT prime and therefore composite.
>
> 6) If a number is not divisible by any of the prime numbers less than
> the square, it is PRIME, if it is, it's COMPOSITE.
>
> I hope this will help you.
>
> -Kartik
>
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