CODEWITHSUNDEEP
phpprimesprime-factoring
Wild Widow
Wild Widow

Reputation: 2549

check whether a number is prime or not?

For checking whether a number is a prime or not, wouldn't be enough to check if it is divisible by 2, 3, 5, and 7? While looking at other programs on the internet, I have found that people are checking the factors till the number, or half of the number or till the square root of the number. 

if ( ($number%2 ==0) || ($number%3==0) || ($number%5==0) || ($number%7)==0) )
    echo "not a prime";

The above check would suffice right? Any thoughts? ignore the prime numbers 2, 3, 5, 7 for now.

Upvotes: 0

Views: 427

Answers (1)

Danil Gaponov
Danil Gaponov

Reputation: 1441

No, it is not enough. For example a prime number 11 is not divisible by 2, 3, 5, and 7. And not a prime number 121 is not divisble by 2, 3, 5, and 7, but is divisible by 11. See the definition of the prime number.

Upvotes: 1

Related Questions