What is the largest prime factor of the number 600851475143 ?

Time：2020-09-10 12:55:33
Class：Weblog
Read：29

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ?

We can start prime number 2 and keep dividing the Number until it can’t, then move to next prime number. Repeat this process until the number becomes 1. Prime number testing can be done in O(Sqrt(N)).

function isPrime(n) {
    if (n == 2 || n == 3) return true;
    for (let i = 2; i * i < n; i ++) {
        if (n % i === 0) return false;
    }
    return true;
}

function isPrime(n) {
    if (n == 2 || n == 3) return true;
    for (let i = 2; i * i < n; i ++) {
        if (n % i === 0) return false;
    }
    return true;
}

Running the following Javascript code to find the largest Prime factor:

function largestPrimeFactor(n) {
    let prime = 2;
    while (n > 1) {
        while (n % prime === 0) {
            n /= prime;
        }
        if (n == 1) break;
        do {
           prime ++;
        } while (!isPrime(prime));
    }
    return prime;
}
 
console.log(largestPrimeFactor(600851475143));

function largestPrimeFactor(n) {
    let prime = 2;
    while (n > 1) {
        while (n % prime === 0) {
            n /= prime;
        }
        if (n == 1) break;
        do {
           prime ++;
        } while (!isPrime(prime));
    }
    return prime;
}

console.log(largestPrimeFactor(600851475143));

The answer is: 6857. As each integer can be represented (factorized) using prime numbers such as 2^a*3^b*5^c…. We can skip prime testing and just use a simple loop to search for the largest prime factor.

function largestPrimeFactor(n) {
    let i = 2;
    while (i * i < n) {
        while (n % i == 0) {
            n /= i;
        }
        i ++;
    }
    return n;
}

function largestPrimeFactor(n) {
    let i = 2;
    while (i * i < n) {
        while (n % i == 0) {
            n /= i;
        }
        i ++;
    }
    return n;
}

–EOF (The Ultimate Computing & Technology Blog) —

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