A factor is an integer that divides another integer evenly. If ^a /_b is an integer, then b is a factor of a. The numbers 3, 4, and 6, for example, are factors of 12.

Sometimes it is necessary or helpful to factor an integer completely. This means you need to find all the factors of that integer. It’s possible that the test will directly require this skill or will make use of it in a more complicated question. In either case, it’s something you should know how to do.

To find all the factors of a number, write them down in pairs, beginning with 1 and the number you’re factoring. We’ll factor 24 in this example. One and 24 are both factors of 24. Next, try every integer greater than 1 in increasing order. Here are the factor pairs we find for 24:

You know you’ve found all the factors of a number when the next first factor exceeds its corresponding second factor. For example, after you found that 4 was a factor of 24 and 5 was not, you would see that 6, the next factor of 24, had already been included in a pair of factors. Thus, all the factors have been found.

A prime number is a number whose only factors are 1 and itself. All prime numbers are positive (because every negative number has –1 as a factor in addition to 1 and itself). Furthermore, all prime numbers besides 2 are odd. The first few primes, in increasing order, are:

To determine whether a number is prime, you shouldn’t check whether the number is divisible by every number less than itself. Such an effort would take an incredible amount of time, and you have only an hour for the Math IC. Instead, to decide whether a number is prime, all you need to do is estimate the square root of the number, then check all the prime numbers that fall below your estimate. For example, to see if 91 is prime, you should estimate the square root of the number: . Now you should test 91 for divisibility by the prime numbers smaller than 10: 2, 3, 5 and 7.

Therefore, 91 is not prime.

Another form of factorization is called prime factorization. The prime factorization of an integer is the listing of the prime numbers whose product is that number.

To find the prime factorization of a number, divide it and all its factors until every remaining integer is prime. This group of prime numbers is the prime factorization of the original integer. As an example, let’s find the prime factorization of 36.

It can be helpful to think of prime factorization in the form of a tree:

As you may already have noticed, there is more than one way to find the prime factorization of a number. We could have first resolved 36 into 6 6, for example, and then determined the prime factorization from there. So don’t worry—you can’t screw up. No matter which path you take, you will always get the same result. That is, as long as you do your arithmetic correctly. Just for practice, find the prime factorizations for 45 and 41.

Since the only factors of 41 are 1 and 41, 41 is a prime number. It is therefore its own prime factorization.

The greatest common factor (GCF) of two numbers is the greatest factor that they have in common. Finding the GCF of two numbers is especially useful in certain applications, such as manipulating fractions (we explain why later in this section).

In order to find the GCF of two numbers, we must first produce their prime factorizations. What is the greatest common factor of 18 and 24, for example?

First, their prime factorizations:

The greatest common factor is the greatest integer that can be written as a product of common prime factors. That is to say, the GCF is the “overlap,” or intersection, of the two prime factorizations. In this case, both prime factorizations contain 2 3 = 6. This is their GCF.

Here’s another example:

First:

So, the product of the prime factors that they share is 2⁴ 3 = 48, which is their GCF.

For practice, find the GCF of the following pairs of integers:

Compare your answers to the solutions:

Two numbers are called relatively prime if they have no common prime factors (i.e., if their GCF is 1). This doesn’t mean, however, that each number is itself prime. The numbers 8 and 15 are relatively prime because they have no common primes in their prime factorizations (8 = 2 2 2 and 15 = 3 5), but neither number is prime.