Math Is Fun Forum

Primenumbers

Member

Registered: 2013-01-22

Posts: 149

Find all the prime numbers in a given range....

Take your Range x.
Delete All even No.'s then take all remaining no.'s and times by 3. Delete these no.'s Repeat for ALL PRIMES less than or = to sqrt.(x).

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"Time not important. Only life important." - The Fifth Element 1997

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: Find all the prime numbers in a given range....

Times by 3?

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

Primenumbers

Member

Registered: 2013-01-22

Posts: 149

Re: Find all the prime numbers in a given range....

Multiplied rather than Times by...? Here is an example:

x=17 sqrt(17)=3 rounded down to the nearest prime.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 (Delete ALL Even No.'s)

  1 3 5 7 9 11 13 15 17 (Multiply remaining no.'s by 3 = 3,9,15,21 etc.) Remove these No.'s Remaining = Primes bar 1

   1 5 7 11 13 17

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"Time not important. Only life important." - The Fifth Element 1997

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: Find all the prime numbers in a given range....

That is incorrect. For example, 25 would remain after the process, but is not prime.

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

Primenumbers

Member

Registered: 2013-01-22

Posts: 149

Re: Find all the prime numbers in a given range....

Say x=29,

sqrt(29)=5 rd. dwn. to nearest prime,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Delete All Even No.'s,
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Times remaining by 3= 3,9,15,21,27,33 etc.......Remove these:
1 5 7 11 13 17 19 23 25 29     Remaining No.'s Timesed by 5 = 5,25,35 etc. Remove theses again = 1 7 11 13 17 19 23 29.

Take all primes < or = to sqrt.(x).

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"Time not important. Only life important." - The Fifth Element 1997

danaj

Member

Registered: 2014-03-03

Posts: 29

Re: Find all the prime numbers in a given range....

What is the advantage of this over the well known Sieve of Eratosthenes?

I think you meant to say "up to a limit" (e.g. find all primes up to 29) instead of "in a given range" (e.g. find all primes in the range 5,208,079 and 6,206,197).

Last edited by danaj (2014-08-28 20:08:19)

Primenumbers

Member

Registered: 2013-01-22

Posts: 149

Re: Find all the prime numbers in a given range....

Yes. But should be able to work out a rule to find out the number of primes up to a limit.

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"Time not important. Only life important." - The Fifth Element 1997