Counting Ways to Place 8 Rooks on a Chess Board without Attacks

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Homework Help Overview

The problem involves determining the number of ways to place 8 identical rooks on an 8x8 chessboard such that no two rooks can attack each other. The participants are exploring the implications of rook placement and the constraints imposed by the chessboard's rows and columns.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Some participants attempt to calculate the number of choices for placing rooks based on available squares, while others question the necessity of dividing by 8! depending on the orientation of the chessboard. There is also a discussion about the geometric implications of rook placement and the relationships between them.

Discussion Status

The discussion is ongoing, with various interpretations of the problem being explored. Some participants have offered insights into the correct reasoning for the placement of rooks, while others express uncertainty about the overlap of attack regions and the total number of valid configurations.

Contextual Notes

Participants are considering the implications of chessboard orientation and the uniqueness of square identification, which may affect the calculation of arrangements. There is also mention of the complexity of the problem and the existing literature on the topic.

cragar
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Homework Statement


How many ways can you place 8 rooks on a chess board so that no 2 rooks attack each other.
Assume the rooks are identical.
Chess board is 8 by 8.

The Attempt at a Solution


To place the first rook I would have 64 choices.
for the second rook I would have 49 choices because I would be limited to a 7 by 7 square.
So the answer should be (64)(49)(36)(25)(16)(9)(4)(1).
I don't think I need to divide by 8! because when I place the rooks that is already taken care of.
 
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cragar said:

Homework Statement


How many ways can you place 8 rooks on a chess board so that no 2 rooks attack each other.
Assume the rooks are identical.
Chess board is 8 by 8.

The Attempt at a Solution


To place the first rook I would have 64 choices.
for the second rook I would have 49 choices because I would be limited to a 7 by 7 square.
So the answer should be (64)(49)(36)(25)(16)(9)(4)(1).
I don't think I need to divide by 8! because when I place the rooks that is already taken care of.

I don't think you need to divide by 8! either, assuming the chess board is oriented so you know which side is black and which white. That way each square is uniquely identified and I agree with your solution. However, if you are just presented with a chess board with no orientation, then you would have to account for rotations which you now couldn't distinguish.
 
cragar said:

Homework Statement


How many ways can you place 8 rooks on a chess board so that no 2 rooks attack each other.
Assume the rooks are identical.
Chess board is 8 by 8.

The Attempt at a Solution


To place the first rook I would have 64 choices.
for the second rook I would have 49 choices because I would be limited to a 7 by 7 square.
So the answer should be (64)(49)(36)(25)(16)(9)(4)(1).
I don't think I need to divide by 8! because when I place the rooks that is already taken care of.

Interesting problem! :smile:

I get 8+8 = 16. Are you sure you can place all 8 rooks using your method?
 
you think the answer is 16? I mean when you place a rook down you eliminate that row and column for your next rook.
 
cragar said:
you think the answer is 16? I mean when you place a rook down you eliminate that row and column for your next rook.

Correct. So what relationship do all the rooks have to have to one another in order not to be able to attack each other?
 
Each rook has to be in a different row and column, So the first rook has 8 choices for the row, then the next rook has 7 choices for the row ect... it seems like the answer might be 8!. Thanks for your help by the way, I am kinda worried about the region where the places the rooks can attack and how that over laps with rooks not in their row or column
 
cragar said:
Each rook has to be in a different row and column, So the first rook has 8 choices for the row, then the next rook has 7 choices for the row ect... it seems like the answer might be 8!. Thanks for your help by the way, I am kinda worried about the region where the places the rooks can attack and how that over laps with rooks not in their row or column

I still get 8+8 = 16. Think geometrically...
 
cragar said:
Each rook has to be in a different row and column, So the first rook has 8 choices for the row, then the next rook has 7 choices for the row ect... it seems like the answer might be 8!. Thanks for your help by the way, I am kinda worried about the region where the places the rooks can attack and how that over laps with rooks not in their row or column

It turns out that a lot of study has gone into this problem. Your thinking here for 8! is correct for an oriented chessboard (which is equivalent to dividing your original calculation by 8!). Google rooks on a chessboard for more information than you will want.
 
After a PM with LCKurtz, I see now that there are more than the 16 positions that I was thinking of. Thanks! :smile:
 

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