[AI] formula for prime numbers

Sandeep Singh sandeepsingh477 at gmail.com
Thu Sep 2 09:03:06 EDT 2010


Yes, one example is 12317, this is a product of 
109 and 113, both prime numbers. Hence even the 
square root method will be very tedious here. 109 
will be some 25th prime number.
Regards,
Sandeep

At 02:31 PM 02-09-10, you wrote:
>Divisibility test for 7
>
>Sir, the divisibility test for 7 is as mentioned below:
>
>• Take the last digit in a number.
>• Double and subtract the last digit in your 
>number from the rest of the digits.
>• Repeat the process for larger numbers.
>• Example: 357 (Double the 7 to get 14. Subtract 14 from 35 to get 21
>which is divisible by 7 and we can now say that 357 is divisible by 7.
>
>Further, we cannot simply use these divisibility tests to determine if
>a No. is prime. For instance, a number might not be divisible by 2, 3,
>4, 5, 6, 7, 8, 9, 10 or 11. However, the same number might be
>divisible by some other No. Therefore, it is imparative that we work
>out all the steps as given in my previous mail.
>
>Regards,
>-Kartik
>
>On 9/2/10, Zujar Shabbir Kanchwala <zujarbright at gmail.com> wrote:
> > Divisibility test for 9:
> >
> > If the one-digit sum total of the digits of a number is 9 then the the
> > number is divisible by 9. For example,
> >
> > Let's take the number 2471913
> > Total of digits = 2 + 4 + 7 + 1 + 9 + 1 + 3 =27
> > once more, total of digits = 2 + 7 = 9
> > Hence, the number 2471913 is divisible by 9.
> >
> > NB: This divisibility test is similar to the one for 3 except that the
> > result for divisibility by 3 should be 3, 6 or 9.
> >
> > hth...
> >
> > --
> > Best Regards,
> > Zujar...
> >
> > An optimist laughs to forget, whereas a pessimist forgets to laugh!
> >
> >
> > On 9/2/10, Amiyo Biswas <amiyo.biswas at gmail.com> wrote:
> >> 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
> >>>
> >>> Voice your thoughts in the blog to discuss the Rights of persons with
> >>> disability bill at:
> >>> http://www.accessindia.org.in/harish/blog.htm
> >>>
> >>> To unsubscribe send a message to accessindia-request at accessindia.org.in
> >>> with the subject unsubscribe.
> >>>
> >>> To change your subscription to digest mode or make any other changes,
> >>> please visit the list home page at
> >>>
> >>> http://accessindia.org.in/mailman/listinfo/accessindia_accessindia.org.in
> >>>
> >>
> >>
> >> Voice your thoughts in the blog to discuss the Rights of persons with
> >> disability bill at:
> >> http://www.accessindia.org.in/harish/blog.htm
> >>
> >> To unsubscribe send a message to accessindia-request at accessindia.org.in
> >> with
> >> the subject unsubscribe.
> >>
> >> To change your subscription to digest mode or make any other changes,
> >> please
> >> visit the list home page at
> >>
> >> http://accessindia.org.in/mailman/listinfo/accessindia_accessindia.org.in
> >>
> >
> > Voice your thoughts in the blog to discuss the Rights of persons with
> > disability bill at:
> > http://www.accessindia.org.in/harish/blog.htm
> >
> > To unsubscribe send a message to 
> accessindia-request at accessindia.org.in with
> > the subject unsubscribe.
> >
> > To change your subscription to digest mode or 
> make any other changes, please
> > visit the list home page at
> >   http://accessindia.org.in/mailman/listinfo/accessindia_accessindia.org.in
> >
>
>Voice your thoughts in the blog to discuss the 
>Rights of persons with disability bill at:
>http://www.accessindia.org.in/harish/blog.htm
>
>To unsubscribe send a message to 
>accessindia-request at accessindia.org.in with the subject unsubscribe.
>
>To change your subscription to digest mode or 
>make any other changes, please visit the list home page at
>   http://accessindia.org.in/mailman/listinfo/accessindia_accessindia.org.in





More information about the AccessIndia mailing list