Georgeooga-Harryooga Theorem

Definition

The Georgeooga-Harryooga Theorem states that if you have $a$ distinguishable objects and $b$ objects are kept away from each other, then there are $\frac{(a-b)!(a-b+1)!}{(a-2b+1)!}$ ways to arrange the objects in a line. Note that you should not use this theorem in a solution if you do not want to get points off.

Created by George and Harry of The Ooga Booga Tribe of The Caveman Society

Proofs

Proof 1

Let our group of $a$ objects be represented like so $1$ , $2$ , $3$ , ..., $a-1$ , $a$ . Let the last $b$ objects be the ones we can't have together.

Then we can organize our objects like so $\square1\square2\square3\square...\square a-b-1\square a-b\square$ .

We have $(a-b)!$ ways to arrange the objects in that list.

Now we have $a-b+1$ blanks and $b$ other objects so we have $_{a-b+1}P_{b}=\frac{(a-b+1)!}{(a-2b+1)!}$ ways to arrange the objects we can't put together.

By The Fundamental Counting Principle our answer is $\frac{(a-b)!(a-b+1)!}{(a-2b+1)!}$ .

Proof by RedFireTruck (talk)

Proof 2

Let us call the $b$ people $1, 2, ... b$

Let the number of people before $1$ in line be $y_1$ , between $1, 2$ be $y_2$ , ... after $b$ be $y_{b+1}$ . We have $y_1 + y_2 + y_3 + \dots y_{b+1} = a-b$

The number of ways to determine $y_1, y_2, \dots$ is equivalent to the number of positive integer solutions to: $x_1 + x_2 + .. + x_{b+1} = a-b + 2$ where $(x_2, ... x_b) = (y_2, ..., y_b)$ and $(x_1, x_{b+1}) = (y_1 +1, y_{b+1} + 1)$ .

So, by stars and bars, the number of ways to determine $(y_2, ..., y_b)$ is $F(a,b) = \dbinom{a-b+1}{b} = \frac {(a-b+1)!}{b!(a-2b+1)!}$

Furthermore, after picking positions for the people, we have $(a-b)!$ ways to order the $(a-b)$ people who can be together, and $b!$ ways to order the $b$ people who cannot be together. So for each $(y_1, y_2, ... y_{b+1}$ , we have $b! (a-b)!$ orderings.

Therefore, the final answer is $b! (a-b)! F(a,b) = \frac{(a-b)!(a-b+1)!}{(a-2b+1)!}$

Proof by Aryabhata000

Problems

Problem 1

Alice, Bob, Carl, David, Eric, Fred, George, and Harry want to stand in a line to buy ice cream. Fred and George are identical twins, so they are indistinguishable. Alice, Bob, and Carl had a serious disagreement in 6th grade, so none of them can be together in the line.

With these conditions, how many different ways can you arrange these kids in a line?

Problem by Math4Life2020

Solution

If Fred and George were distinguishable we would have $\frac{(8-3)!(8-3+1)!}{(8-2\cdot3+1)!}=14400$ ways to arrange them by the Georgeooga-Harryooga Theorem. However, Fred and George are indistinguishable so we have to divide by $2!=2$ . Therefore, our answer is $\frac{14400}2=\boxed{7200}$ .

Solution by RedFireTruck (talk)

Problem 2

Zara has a collection of $4$ marbles: an Aggie, a Bumblebee, a Steelie, and a Tiger. She wants to display them in a row on a shelf, but does not want to put the Steelie and the Tiger next to one another. In how many ways can she do this?

$\textbf{(A) }6 \qquad \textbf{(B) }8 \qquad \textbf{(C) }12 \qquad \textbf{(D) }18 \qquad \textbf{(E) }24$

(Source)

Problem 3

The $7$ members of The Ooga Booga Tribe, Lord Ooga Booga, Ooga, Booga, Foogle, Hoogle, George, and Harry, are about to perform a ritual. They have invited $2$ priests, Agoob and Agoo, from a neighboring tribe. In this ritual they will line up in a row and sit down. The $2$ priests must sit next to each other. Lord Ooga Booga, Ooga, and Booga just had a family argument so they must stay away from each other. In how many ways can The Ooga Booga Tribe perform their ritual?

Problem by RedFireTruck

Solution

Let Agoob and Agoo be one object called Agooboo. Then, by the Georgeooga-Harryooga Theorem there are $\frac{(8-3)!(8-3+1)!}{(8-2\cdot3+1)!}=14400$ ways to arrange the $7$ members and Agooboo. However, there are $2$ ways to "split" Agooboo. So, by the Fundamental Counting Principle, our answer is $14400\cdot2=\boxed{28800}$ .

Solution by RedFireTruck (talk)

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Georgeooga-Harryooga Theorem

Contents

Definition

Proofs

Proof 1

Proof 2

Problems

Problem 1

Solution

Problem 2

Problem 3

Solution