How to check whether a given number like 3157 is prime or not?

Answer 1

It isn't
11 * 287 = 3157

Answer 2

There is no short cut methods. You have to devide by all the primes upto square root of the number or divisible by a number where you stop. Again if the number is very large you cannot know all the primes till square root. So devide by 2 and then 3 then 6n-1 and 6n+1 till you reach the end of check

There may be short cut for some number division

say for 3157. last digit 7 no not divisible by 2
sum of digits = 16 not divisible by 3

not by 5

but divide by 7 it devides 7*431
so this is not prime

Answer 3

TRy dividing it by any number other than one and itself. 3157 is divisible by 7, 11 and 41; hence not a prime number.

Answer 4

3157 isn't a Prime Number Buddy

Answer 5

to check a given number mentally i have a few handy rule:

divisible by 2 : any even number
div by 3: add all the digits together to see if it is div by 3.
ie for 3157, add 3+1+5+7=16 so 3157 is not divisible by 3
div by 4: check the last 2 digits. if last 2 digits is divisible by 4 then the number is div by 4
div by 5: this is easy. last digit is 0 or 5.
div by 11: add odd-placed digits, add even-placed digits. if they are equal, then it is div by 11.
ie: for 3157, add 3+5 compare to 1+7. so 3157 is div by 11

i dont know methods for div by 7,13,17,19 sorry.

Answer 6

There is an algorithm called the "Sieve of Eratosthenes", which was invented by the same Greek fellow that first measured the radius of the Earth. It can be used to determine any prime number. Here's how it works:

Factors are integers larger than zero and smaller than the number whose product equals the number.

Since any number times one is itself, one is not considered a factor.

Any number that ends with an even number (0,2,4,6,8) is a multiple of two and is not prime. This test is so simple that it's a worthwhile shortcut. It also saves you from testing any even-valued factors because all even numbers are divisible by two.

Any non-prime number will have factors that are less than or equal to its square root; for example sqrt(9) is three, and 3*3=9. Any factor larger than the square root must be multiplied by a number smaller than the square root or their product will be larger than the target number. Therefore, it's only necessary to search between 3 and the square root for factors.

Let's look at 49 for example. Root 49 is 7, so we need to example 3, 5, and 7. 49/3 is not an integer so 3 is not a factor. 49/5 is not an integer either so 5 is not a factor. 7 is the square root, so it is a factor and it is also the largest factor.

Answer 7

I am not quite sure about this... try searching in google something like:
'Is 3157 prime' sorry i could not be of any more help

Answer 8

On contemporary computers, with the ideal universal contemporary algorithms, attempting out the primality of an arbitrary 2 hundred-digit variety is trivial, yet a one thousand-digit variety is a severe undertaking and 2000 digits is in result quite impracticable. Trial branch as much as the sq. root is the finest yet least effective set of rules universal. record-breaking Mersenne primes have a particular type which permits using a specifically effective particular attempt.

Answer 9

A prime number is only evenly divisible by one and itself. This number is not a prime because it can be divided bu 11.

3157/11=287.

Answer 10

take the square root the given no
if this square root is divisible by any prime no then the given is not prime