Question

If you want to examine if a integer is a prime number can you try with dividing it by
the prime numbers 2 , 3, 5 etc . What is the largest prime number that there is reason to try ?
Determine if any of the numbers 171, 203 , 211, 567 or 7669 is prime and factorize them otherwise.

Answer 1

the number's square root is a cut off point

Answer 2

A short cut to see if a number is divisible 3 is to add the digits up until you have 1 didigte
If this digit is 3 , 6 or 9 then t is divisible by 3.

Answer 3

so from this you can see that 171 and 567 are not prime

Answer 4

square root of 203 is 14.25 so you keep dividing by prime numbers up to and including 13.

Answer 5

It is sort of fun to think about why the square root thing works.

Answer 6

@@zzr0ck3r could you explain why that works? is it because everything after the squareroot is like testing the dividers all over again?

Answer 7

I think it works because if there is an integer quotient you would have found it already when dividing by the lower primes.

Answer 8

- yes - it is like testing the dividers all over again

Answer 9

well lets take any number say x
the biggest factor of any number can be the square root of the number itself :) when u prime factorize x u will get prime numbers as its factors
so the prime factors of any number x are always smaller than or equal to root{x}
if x is a prime number then there will be no factors but if x is composite then the factors it will have will always be less than or equal to root(x)

so to check if x is prime we find root x nd then try dividing x by all prime numbers less then rootx

Answer 10

so if Im gonna see if 7669 is a prime, how do i do? the sqrt of 7669 is 87.6, then still I gotta know the primes upp to that

Answer 11

@zzr0ck3r , @welshfella , @imqwerty

Answer 12

yes

Answer 13

yes 2,3,5,7 etc

Answer 14

up to 83

Answer 15

thats correct
because square root of 7669 is 87.57

Answer 16

oh you found that already - sorry

Answer 17

is 211 a prime?

Answer 18

no, 3*3*19

Answer 19

that is 171