The Knights Dilemma

What is the maximum number of knights you can place on a 8 × 8 8 \times 8 chessboard such that no two of them attack each other?

Details and assumptions

  • The knights are placed on distinct cells.

  • Two knights attack each other if they are one cell vertically and two cells horizontally apart or two cells vertically and one cell horizontally apart.

  • A 8 × 8 8 \times 8 chessboard has 8 2 = 64 8^2= 64 cells.

  • Here's an example of 2 2 knights being placed on a 3 × 3 3 \times 3 chessboard which attack each other.

Image credit: Wikipedia


The answer is 32.

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6 solutions

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We can divide the chessboard into 32 groups each with two squares(I only coloured 8 groups, you can colour the rest using the same pattern.) such that the squares are a knight's move away.

Let the number of knights that can be placed be n n . If n > 32 n > 32 , then by PHP, two knights must be in the same group. But then they will attack each other. Therefore n 32 n \leq 32 . To prove that n = 32 n = 32 is the maximum, we must find a configuration of 32 knights.

Note that a knight always attacks squares of a different color.(i.e. if its on a black square, it attacks only white squares.) Therefore, if we put all the knights on squares of the same color(WLOG black), then they will not be able to attack each other. Total number of black squares = 32.

Therefore the maximum number of knights which can be placed on a 8X8 chessboard so that they do not attack each other is 32 \boxed{32} .

Thanks for the clear solution

Jonathan Moey - 7 years, 1 month ago

what's PHP?

Sahil Gohan - 7 years, 1 month ago

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Pigeonhole Principle.

Sharky Kesa - 6 years, 10 months ago
Dpk ­
Mar 16, 2014

If you play chess you would know that a knight can only attack or move to a square of the opposite color as the square where it came from (so it's always Black -> White and White -> Black). If you put all knights on the same square color then they can't attack each other. Since in a 8x8 board there are 32 white squares and 32 black squares, then the maximum number of knights is 32

But how do you know that there isn't an arrangement which would allow you to place more than 32 knights?

Calvin Lin Staff - 7 years, 2 months ago

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There isn't any arrangement that can give more than 32 knights, its is the maximum number, atleast on a 8 by 8 chess board

Kushagra Sahni - 7 years ago

one on alternate space

Dushyant Tanna - 7 years, 2 months ago
Hamada Ragab
Mar 21, 2014

Simply the knight must move to the other color, if it was on white it can't attack white, so you have 32 white squares you can place them all.

hahahh

Daisy Luzon - 7 years, 2 months ago
Ladu Aadarsh
Jun 4, 2014

The knights always attack each other only on squares of opposite colours. so place all the knights on the squares of the same colour. there are 32 squares of the same colour on the chessboard. So the answer is 32

Gunjas Singh
May 27, 2014

A knight , if placed on white square can only attack on a black one OR

if placed on a white square can only attack black square

so, maximum number of knights which can b placed on a 8 X 8 chessboard would either be the number of black squares or the number of white squares( becauz if not to attack each other, they have to lie in the same color) therefore, the answer is (8 X 8)/2 (i.e. the number of black/white squares)

Viswanath Reddy
Mar 12, 2014

4 in each row..one in every 2 cells of a row...

How do you ensure that this is the optimal arrangement?

Sreejato Bhattacharya - 7 years, 3 months ago

Brother,i didnt think this in combinatorics way or any related things.I had a few ideas and tried for an arrangement with minimal spacing.a knight in an edge row influences knights of its row and adjacent 2 rows.a knight in the row adjacent to edge row influences knights of total 4 rows.A knight in any other row influences knights of 5 rows,So,i thought of 5*5 box.And if 2 knights are adjacent to each other,spaces are more than when they are separated by a cell. With such ideas,i come to this arrangement..Please tell me how you solve this?

Viswanath Reddy - 7 years, 3 months ago

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A knight always attacks squares of a different color.(Eg, a knight on a white square attacks only black squares.). So putting all the knights on only blacks squares or only white squares ensures no knight attacks another. Total number of squares of same color = 32.

Siddhartha Srivastava - 7 years, 3 months ago

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