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Relatively Prime Numbers

What is Relatively Prime?

Relatively prime is a term that is seemingly misleading. The number 15 is relatively prime to 16, but neither 15 nor 16 is prime. By definition, two numbers are relatively prime if and only if the greatest common divisor of both numbers is 1. The most common type of problem found on number sense tests involving relatively prime numbers is “How many positive numbers less than or equal to x are relatively prime to x ?”

Number of positive numbers less than x Relatively Prime to x

To find the number of positive numbers less than x that are relatively prime to x , follow these steps:

  • Find the prime factorization of x in the form of where p i is a unique prime factor of x and n i is the power of prime p i found in x .
  • Then for each prime number pi ; (1 i k ), create two new numbers Ai and Bi . Ai = pi - 1 and . Finally, the number of positive integers less than or equal to x and relatively prime to x is determined by finding the product of all Ai · Bi ; (1 i k ). The following following is a generalization.

So, in other words, if x = , then the number of positive numbers less than or equal to x that are relatively prime to x is

So,... what does that mean?

Without all the scary math symbols, here’s basically what you have to do:

For each prime factor raised to some power, find the number one less than the prime and the number that is the prime raised to a power that is one less than the original power.

I think some examples will be helpful.

Example:

How many positive numbers less than or equal to 15 are relatively prime to 15?

First, factor 15 into its primes: 15 = 3 1 · 5 1

Then, use the formula above:

For 3 1 , we get 3 - 1 = 2 and 3 1 - 1 = 3 0 = 1 (Every positive number raised to the zero power is 1.)

Also from 5 1 , we get 5 - 1 = 4 and 5 1 - 1 = 5 0 = 1.

Multiply all the new numbers together to get the answer. 2 × 1 × 4 × 1 = 8.

 

This example was easy because every prime has a power of 1. When this is the case, you can simply multiply the numbers one less than the primes to find the number of positive integers less than x that are relatively prime.

Example:

How many positive numbers less than or equal to 16 are relatively prime to 16?

First, factor 16 into its primes: 16 = 2 4

Then, use the formula:

For 2 4 , we get 2 - 1 = 1 and 2 4 - 1 = 2 3 = 8.

Multiply these two numbers together to get the answer. 1 × 8 = 8.

Example:

How many positive numbers less than or equal to 144 are relatively prime to 144?

Factor 144 = 2 4 × 3 2 .

Use the formula for each prime:

From 2 4 , we get 2 - 1 = 1 and 2 4 - 1 = 2 3 = 8.

From 3 2 , we get 3 - 1 = 2 and 3 2 - 1 = 3 1 = 3.

Multiply these numbers together to get the answer. 1 × 8 × 2 × 3 = 48.

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