i found on the internet a formula that said if the result of 6n+1 or 6n-1 is a natural number where n is the number you wish to check the n must be prime

ie 7 - 1 = 6/6 = 1 7 is prime.

carrying this forward lets take the number 13195 the above states its a prime number. which is wrong

question is rather than checking every number from 2 to n how do you calculate a prime