Identical objects in permutations

Definition

Identical objects in permutations refer to scenarios where some of the items being arranged are indistinguishable from one another, making the arrangement of these items unique. When dealing with identical objects, the total number of permutations is reduced because swapping identical objects does not create a new arrangement. This concept is crucial for accurately calculating the number of distinct arrangements when certain items cannot be distinguished from each other.

5 Must Know Facts For Your Next Test

1. When calculating permutations with identical objects, the formula used is $$rac{n!}{n_1! imes n_2! imes ...}$$ where $$n$$ is the total number of items, and $$n_1, n_2, ...$$ are the counts of each type of identical object.
2. The presence of identical objects simplifies many problems by reducing the number of unique arrangements that can be formed.
3. Identical objects can appear in various contexts, such as letters in a word or colored balls in a box, affecting how we count arrangements.
4. In cases with no identical objects, the total number of permutations is simply calculated as $$n!$$.
5. Understanding how to account for identical objects helps avoid overcounting arrangements that are actually the same.

Review Questions

• How does the presence of identical objects affect the total number of permutations when arranging a set of items?
  • The presence of identical objects decreases the total number of unique permutations compared to a set with all distinct items. This is because swapping identical items does not create a new arrangement. To calculate permutations accurately, we use the formula $$\frac{n!}{n_1! \times n_2! \times ...}$$ where $$n$$ is the total number of items and $$n_1, n_2,...$$ represent counts of each type of identical object.
• What is the formula for calculating permutations when dealing with identical objects and how do you apply it in a practical example?
  • The formula for calculating permutations with identical objects is $$\frac{n!}{n_1! \times n_2! \times ...}$$. For example, if you have the word 'BALLOON', there are 7 letters total with 'L' appearing twice and 'O' appearing twice. The calculation would be $$\frac{7!}{2! \times 2!}$$ which accounts for the repeated letters to give you the correct number of unique arrangements.
• Evaluate a real-world situation where you might need to apply your understanding of identical objects in permutations and describe how you would solve it.
  • Consider organizing a group photo with 5 people where 2 are wearing identical shirts. To find out how many distinct arrangements can be made, I would use the formula $$\frac{5!}{2!}$$ since the two shirts are identical. This approach accounts for indistinguishability and gives me an accurate count of different photo setups possible. In this case, I would calculate $$\frac{120}{2} = 60$$ distinct arrangements.

Related terms

Permutations: Arrangements of objects where the order matters, often calculated using factorial notation to represent all possible arrangements.

Factorial: A mathematical operation that multiplies a number by all positive integers below it, denoted as n!, used to calculate permutations and combinations.

Combinations: Selections of items where the order does not matter, differentiating it from permutations which consider the arrangement of items.