Is it always possible to fit these pieces in a square?
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Consider all possible pairs of squares that can fit in a row of length $n$ where every square has a width of 1. If I have a large square of width $n$, can all such pairs of squares fit in the large square simultaneously?
It's hard to explain without an example so here is the case when $n=4$:
To the left are all the pairs of squares and to the right is a way for them to fit in the $n$ by $n$ square. Note that the pairs are not allowed to be moved horizontally but can be moved vertically.
It becomes a little harder but still possible when $n=5$:
What I'm interested in is the general case. Is it possible to fit all pairs in the square for any $n$?
recreational-mathematics
New contributor
|
show 3 more comments
up vote
8
down vote
favorite
Consider all possible pairs of squares that can fit in a row of length $n$ where every square has a width of 1. If I have a large square of width $n$, can all such pairs of squares fit in the large square simultaneously?
It's hard to explain without an example so here is the case when $n=4$:
To the left are all the pairs of squares and to the right is a way for them to fit in the $n$ by $n$ square. Note that the pairs are not allowed to be moved horizontally but can be moved vertically.
It becomes a little harder but still possible when $n=5$:
What I'm interested in is the general case. Is it possible to fit all pairs in the square for any $n$?
recreational-mathematics
New contributor
What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
1
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago
|
show 3 more comments
up vote
8
down vote
favorite
up vote
8
down vote
favorite
Consider all possible pairs of squares that can fit in a row of length $n$ where every square has a width of 1. If I have a large square of width $n$, can all such pairs of squares fit in the large square simultaneously?
It's hard to explain without an example so here is the case when $n=4$:
To the left are all the pairs of squares and to the right is a way for them to fit in the $n$ by $n$ square. Note that the pairs are not allowed to be moved horizontally but can be moved vertically.
It becomes a little harder but still possible when $n=5$:
What I'm interested in is the general case. Is it possible to fit all pairs in the square for any $n$?
recreational-mathematics
New contributor
Consider all possible pairs of squares that can fit in a row of length $n$ where every square has a width of 1. If I have a large square of width $n$, can all such pairs of squares fit in the large square simultaneously?
It's hard to explain without an example so here is the case when $n=4$:
To the left are all the pairs of squares and to the right is a way for them to fit in the $n$ by $n$ square. Note that the pairs are not allowed to be moved horizontally but can be moved vertically.
It becomes a little harder but still possible when $n=5$:
What I'm interested in is the general case. Is it possible to fit all pairs in the square for any $n$?
recreational-mathematics
recreational-mathematics
New contributor
New contributor
edited 2 days ago
Jens
3,5672828
3,5672828
New contributor
asked 2 days ago
Pazzaz
435
435
New contributor
New contributor
What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
1
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago
|
show 3 more comments
What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
1
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago
What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
1
1
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago
|
show 3 more comments
2 Answers
2
active
oldest
votes
up vote
5
down vote
accepted
It is always possible, we can place the $binom{n}{2}$ pairs in a $n times n$ square when $n$ is odd and in a $(n-1) times n$ rectangle when $n$ is even.
This problem is equivalent to the edge coloring problem for complete graph $K_n$. Look at wiki for the geometric intuition underlying following construction.
Let $[n]$ be a short hand for ${ 0, ldots, n-1 }$.
Index the set of possible pairs by $(i,j) in [n]^2$ with $i < j$.
Label rows and columns of the large square using numbers from $[n]$.
When $n$ is odd, place the pair $(i,j)$ at row $k$ of the large square where $i + j equiv k pmod n$.
If two pairs $(i_1,j_1)$, $(i_2,j_2)$ on same row intersect, then
one of the following happens
$$i_1 = i_2 lor i_1 = j_2lor j_1 = i_2 lor j_1 = j_2$$
Since $i_1 + j_1 equiv i_2 + j_2 pmod n$, we find
$$(i_1,j_1) = (i_2,j_2) pmod n lor (i_1,j_1) = (j_2,i_2) pmod n$$
Since $i_1,i_2,j_1,j_2 in [n]$ and $i_1 < j_1$, $i_2 < j_2$, we can rule out the second case. From this, we can deduce distinct pairs on some row are disjoint. This generate
a desired packing of the $binom{n}{2}$ pairs into a $n times n$ square.
When $n$ is even, $n - 1$ is odd.
Place those pair $(i,j) in [n-1]^2$ into row $k$ where $i + j equiv k pmod {n-1}$.
Notice
- For each row $i in [n-1]$, the slot at column $2i pmod {n - 1}$ and $n-1$ is unused.
- For any column $j in [n-1]$, one and only slot at row $i in [n-1]$ is unused.
For those pair $(i,j) in [n]^2 setminus [n-1]^2$ with $i < j$, we have $j = n$.
We can place the pair on row $k$ where $2k = i pmod {n-1}$. This will fill all the unused slots in the first $n-1$ rows and generate a desired packing of the $binom{n}{2}$ pairs into a $(n-1) times n$ rectangle.
add a comment |
up vote
1
down vote
For 7×7, we have the following. X is an empty space, A through U are the 21 pairs of squares. For example, D D in the second row indicates that the {1,3} pair is used in that row, while the X in the seventh position means that spot is left empty.
A A B B C X C
D E D E F F X
G H X I H I G
J K K J X L L
X M N O O M N
P X Q R Q P R
S T U X S U T
.
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
add a comment |
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
5
down vote
accepted
It is always possible, we can place the $binom{n}{2}$ pairs in a $n times n$ square when $n$ is odd and in a $(n-1) times n$ rectangle when $n$ is even.
This problem is equivalent to the edge coloring problem for complete graph $K_n$. Look at wiki for the geometric intuition underlying following construction.
Let $[n]$ be a short hand for ${ 0, ldots, n-1 }$.
Index the set of possible pairs by $(i,j) in [n]^2$ with $i < j$.
Label rows and columns of the large square using numbers from $[n]$.
When $n$ is odd, place the pair $(i,j)$ at row $k$ of the large square where $i + j equiv k pmod n$.
If two pairs $(i_1,j_1)$, $(i_2,j_2)$ on same row intersect, then
one of the following happens
$$i_1 = i_2 lor i_1 = j_2lor j_1 = i_2 lor j_1 = j_2$$
Since $i_1 + j_1 equiv i_2 + j_2 pmod n$, we find
$$(i_1,j_1) = (i_2,j_2) pmod n lor (i_1,j_1) = (j_2,i_2) pmod n$$
Since $i_1,i_2,j_1,j_2 in [n]$ and $i_1 < j_1$, $i_2 < j_2$, we can rule out the second case. From this, we can deduce distinct pairs on some row are disjoint. This generate
a desired packing of the $binom{n}{2}$ pairs into a $n times n$ square.
When $n$ is even, $n - 1$ is odd.
Place those pair $(i,j) in [n-1]^2$ into row $k$ where $i + j equiv k pmod {n-1}$.
Notice
- For each row $i in [n-1]$, the slot at column $2i pmod {n - 1}$ and $n-1$ is unused.
- For any column $j in [n-1]$, one and only slot at row $i in [n-1]$ is unused.
For those pair $(i,j) in [n]^2 setminus [n-1]^2$ with $i < j$, we have $j = n$.
We can place the pair on row $k$ where $2k = i pmod {n-1}$. This will fill all the unused slots in the first $n-1$ rows and generate a desired packing of the $binom{n}{2}$ pairs into a $(n-1) times n$ rectangle.
add a comment |
up vote
5
down vote
accepted
It is always possible, we can place the $binom{n}{2}$ pairs in a $n times n$ square when $n$ is odd and in a $(n-1) times n$ rectangle when $n$ is even.
This problem is equivalent to the edge coloring problem for complete graph $K_n$. Look at wiki for the geometric intuition underlying following construction.
Let $[n]$ be a short hand for ${ 0, ldots, n-1 }$.
Index the set of possible pairs by $(i,j) in [n]^2$ with $i < j$.
Label rows and columns of the large square using numbers from $[n]$.
When $n$ is odd, place the pair $(i,j)$ at row $k$ of the large square where $i + j equiv k pmod n$.
If two pairs $(i_1,j_1)$, $(i_2,j_2)$ on same row intersect, then
one of the following happens
$$i_1 = i_2 lor i_1 = j_2lor j_1 = i_2 lor j_1 = j_2$$
Since $i_1 + j_1 equiv i_2 + j_2 pmod n$, we find
$$(i_1,j_1) = (i_2,j_2) pmod n lor (i_1,j_1) = (j_2,i_2) pmod n$$
Since $i_1,i_2,j_1,j_2 in [n]$ and $i_1 < j_1$, $i_2 < j_2$, we can rule out the second case. From this, we can deduce distinct pairs on some row are disjoint. This generate
a desired packing of the $binom{n}{2}$ pairs into a $n times n$ square.
When $n$ is even, $n - 1$ is odd.
Place those pair $(i,j) in [n-1]^2$ into row $k$ where $i + j equiv k pmod {n-1}$.
Notice
- For each row $i in [n-1]$, the slot at column $2i pmod {n - 1}$ and $n-1$ is unused.
- For any column $j in [n-1]$, one and only slot at row $i in [n-1]$ is unused.
For those pair $(i,j) in [n]^2 setminus [n-1]^2$ with $i < j$, we have $j = n$.
We can place the pair on row $k$ where $2k = i pmod {n-1}$. This will fill all the unused slots in the first $n-1$ rows and generate a desired packing of the $binom{n}{2}$ pairs into a $(n-1) times n$ rectangle.
add a comment |
up vote
5
down vote
accepted
up vote
5
down vote
accepted
It is always possible, we can place the $binom{n}{2}$ pairs in a $n times n$ square when $n$ is odd and in a $(n-1) times n$ rectangle when $n$ is even.
This problem is equivalent to the edge coloring problem for complete graph $K_n$. Look at wiki for the geometric intuition underlying following construction.
Let $[n]$ be a short hand for ${ 0, ldots, n-1 }$.
Index the set of possible pairs by $(i,j) in [n]^2$ with $i < j$.
Label rows and columns of the large square using numbers from $[n]$.
When $n$ is odd, place the pair $(i,j)$ at row $k$ of the large square where $i + j equiv k pmod n$.
If two pairs $(i_1,j_1)$, $(i_2,j_2)$ on same row intersect, then
one of the following happens
$$i_1 = i_2 lor i_1 = j_2lor j_1 = i_2 lor j_1 = j_2$$
Since $i_1 + j_1 equiv i_2 + j_2 pmod n$, we find
$$(i_1,j_1) = (i_2,j_2) pmod n lor (i_1,j_1) = (j_2,i_2) pmod n$$
Since $i_1,i_2,j_1,j_2 in [n]$ and $i_1 < j_1$, $i_2 < j_2$, we can rule out the second case. From this, we can deduce distinct pairs on some row are disjoint. This generate
a desired packing of the $binom{n}{2}$ pairs into a $n times n$ square.
When $n$ is even, $n - 1$ is odd.
Place those pair $(i,j) in [n-1]^2$ into row $k$ where $i + j equiv k pmod {n-1}$.
Notice
- For each row $i in [n-1]$, the slot at column $2i pmod {n - 1}$ and $n-1$ is unused.
- For any column $j in [n-1]$, one and only slot at row $i in [n-1]$ is unused.
For those pair $(i,j) in [n]^2 setminus [n-1]^2$ with $i < j$, we have $j = n$.
We can place the pair on row $k$ where $2k = i pmod {n-1}$. This will fill all the unused slots in the first $n-1$ rows and generate a desired packing of the $binom{n}{2}$ pairs into a $(n-1) times n$ rectangle.
It is always possible, we can place the $binom{n}{2}$ pairs in a $n times n$ square when $n$ is odd and in a $(n-1) times n$ rectangle when $n$ is even.
This problem is equivalent to the edge coloring problem for complete graph $K_n$. Look at wiki for the geometric intuition underlying following construction.
Let $[n]$ be a short hand for ${ 0, ldots, n-1 }$.
Index the set of possible pairs by $(i,j) in [n]^2$ with $i < j$.
Label rows and columns of the large square using numbers from $[n]$.
When $n$ is odd, place the pair $(i,j)$ at row $k$ of the large square where $i + j equiv k pmod n$.
If two pairs $(i_1,j_1)$, $(i_2,j_2)$ on same row intersect, then
one of the following happens
$$i_1 = i_2 lor i_1 = j_2lor j_1 = i_2 lor j_1 = j_2$$
Since $i_1 + j_1 equiv i_2 + j_2 pmod n$, we find
$$(i_1,j_1) = (i_2,j_2) pmod n lor (i_1,j_1) = (j_2,i_2) pmod n$$
Since $i_1,i_2,j_1,j_2 in [n]$ and $i_1 < j_1$, $i_2 < j_2$, we can rule out the second case. From this, we can deduce distinct pairs on some row are disjoint. This generate
a desired packing of the $binom{n}{2}$ pairs into a $n times n$ square.
When $n$ is even, $n - 1$ is odd.
Place those pair $(i,j) in [n-1]^2$ into row $k$ where $i + j equiv k pmod {n-1}$.
Notice
- For each row $i in [n-1]$, the slot at column $2i pmod {n - 1}$ and $n-1$ is unused.
- For any column $j in [n-1]$, one and only slot at row $i in [n-1]$ is unused.
For those pair $(i,j) in [n]^2 setminus [n-1]^2$ with $i < j$, we have $j = n$.
We can place the pair on row $k$ where $2k = i pmod {n-1}$. This will fill all the unused slots in the first $n-1$ rows and generate a desired packing of the $binom{n}{2}$ pairs into a $(n-1) times n$ rectangle.
edited yesterday
answered yesterday
achille hui
93.4k5127251
93.4k5127251
add a comment |
add a comment |
up vote
1
down vote
For 7×7, we have the following. X is an empty space, A through U are the 21 pairs of squares. For example, D D in the second row indicates that the {1,3} pair is used in that row, while the X in the seventh position means that spot is left empty.
A A B B C X C
D E D E F F X
G H X I H I G
J K K J X L L
X M N O O M N
P X Q R Q P R
S T U X S U T
.
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
add a comment |
up vote
1
down vote
For 7×7, we have the following. X is an empty space, A through U are the 21 pairs of squares. For example, D D in the second row indicates that the {1,3} pair is used in that row, while the X in the seventh position means that spot is left empty.
A A B B C X C
D E D E F F X
G H X I H I G
J K K J X L L
X M N O O M N
P X Q R Q P R
S T U X S U T
.
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
add a comment |
up vote
1
down vote
up vote
1
down vote
For 7×7, we have the following. X is an empty space, A through U are the 21 pairs of squares. For example, D D in the second row indicates that the {1,3} pair is used in that row, while the X in the seventh position means that spot is left empty.
A A B B C X C
D E D E F F X
G H X I H I G
J K K J X L L
X M N O O M N
P X Q R Q P R
S T U X S U T
.
For 7×7, we have the following. X is an empty space, A through U are the 21 pairs of squares. For example, D D in the second row indicates that the {1,3} pair is used in that row, while the X in the seventh position means that spot is left empty.
A A B B C X C
D E D E F F X
G H X I H I G
J K K J X L L
X M N O O M N
P X Q R Q P R
S T U X S U T
.
edited 2 days ago
answered 2 days ago
Oscar Lanzi
11.6k11935
11.6k11935
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
add a comment |
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
1
1
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
It looks like for odd n, the empty spots are all in different columns. In other words, by rearranging the rows you can create a blank diagonal.
– Pazzaz
2 days ago
add a comment |
Pazzaz is a new contributor. Be nice, and check out our Code of Conduct.
Pazzaz is a new contributor. Be nice, and check out our Code of Conduct.
Pazzaz is a new contributor. Be nice, and check out our Code of Conduct.
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What does "all possible pieces (disconnected or connected) of width 2 and height of 1 that could fit in the square" mean? Can you show us how to generate the pieces for a given square?
– John Douma
2 days ago
@JohnDouma Well a piece here simply consists of choosing two squares in a single row of length n. So the total number of pieces will be (n choose 2).
– Pazzaz
2 days ago
1
I think you have to specify the problem more accurately. A triangle can have width $2$ and height $1$, for example, and that seems to be different from what you mean (and that is without thinking about figures which are not convex). I think you mean figures constructed of two $1times 1$ squares in a particular kind of configuration (as in your comment). And these sub-figures need not be connected. I do think your intended question is interesting, and worth asking, but the way you have asked it is misleading.
– Mark Bennet
2 days ago
@MarkBennet I agree, I did leave out some important parts. I've edited the question some, it should be more clear now.
– Pazzaz
2 days ago
Well, if possible, there will always be $n^2-2binom n2=n$ leftover spaces.
– YiFan
2 days ago