Discrete math-counting permutation

In summary: So if you had the beads in a line, then you would have 2*(2-1)=2 arrangements.In summary, there are three possible arrangements.
  • #1


Can someone help me understand this one? The problem is: Four beads-red,blue,yellow, and green-are arranged on a string to make a simple necklace as shown in the figure. How many arrangements are possible?

The answer in the book is 3, but I don't get it.

I thought it woud be a permutation because order does matter. So I assume that since there are 4 objects and 4 blanks to fill the permutation counting formula would be (4!)/(4-4)!
Therefore this would equal 4*3*2*1/1=24

The book says the answer is three...am I doing something wrong?

HELLLP :yuck:
Physics news on Phys.org
  • #2
Remember the arrangement Red, blue, yellow, green is exactly the same as the arrangement green, red, blue, yellow. Why? Because it's a necklace. It's circular. There really is not "first" element. Since it's a circle, you can call any element the first element you want. It doesn't change the order.

If it's still unclear, take a simpler example. Say you only have 2 items on your necklace, a car and a house (for whatever reason). How many different arrangements do you have?

Well we have car, house
and we have house,car
except house, car is exactly the same as car, house. We can just shift the necklace a little bit so we start counting the house first. It doesn't actually change the order or arrangement of it.
Last edited:
  • #3
To continue what KoGs was saying: If you had the beads in a line, then
RBYG would be different from BYGR. But if the beads are on a circular string, making a necklace, then RBYG and BYGR would be exactly the same- you just started listing from a different point. Do this: first select anyone of the beads, say R, to put on the necklace. NOW how many choices do you have for the second bead to put on? The third? The fourth? How many choices did you have altogether?

1. What is discrete math-counting permutation?

Discrete math-counting permutation is a branch of mathematics that deals with counting and arranging objects in a specific order, where each object can only be used once.

2. What is the difference between permutation and combination?

Permutation is the arrangement of objects in a specific order, where the order matters. Combination is the selection of objects without considering the order.

3. How do I calculate the number of permutations?

The number of permutations can be calculated using the formula n! / (n-r)!, where n is the total number of objects and r is the number of objects being arranged.

4. What is a factorial?

A factorial is a mathematical function denoted by an exclamation mark (!) that represents the product of all positive integers less than or equal to a given number. For example, 5! = 5 x 4 x 3 x 2 x 1 = 120.

5. How is discrete math-counting permutation used in real life?

Discrete math-counting permutation is used in various fields such as computer science, statistics, and genetics. It is used to analyze and solve various problems related to arrangements and combinations, such as password cracking, DNA sequencing, and data encryption.

Suggested for: Discrete math-counting permutation