Set that doesn't shatter certain subsets
$begingroup$
Let $Asubset mathcal{P}({1, dots, n}) $ and $ B subset {1, dots, n }$
We say $A$ shatters $B$ if $forall y subset B, exists x in A$ such that $x cap B = y$.
I am asked to show that if $A$ does not shatter the sets: ${1,2,3},{2,3,4}, dots, {n-2,n-1,n,}, {n-1,n,1}, {n,1,2}$ and $n$ is a multiple of $3$ then $|A| leq 7^{frac{n}{3}}$
My current thinking is that, for each of these $3$-sets, we have to miss at least one of their subsets.
Specifically, for each $a subset {x,y,z}$ there are $2^{n-3}$ subsets of ${1,dots,n}$ that intersect with ${x,y,z}$ to give $a$. (Call the set of there $2^{n-3}$ subsets $C_{{x,y,z}}(a)$) Hence if $A$ does not shatter ${1,2,3}$ because we are missing $a$, then $A$ cannot contain these $2^{n-3}$ subsets.
I want to say that there is some subset $B subset mathcal{P}({1,dots,n}$ of size $8* 7^{frac{n}{3}}$ such that we must have $A subset B$, and we may only have at most $frac{7}{8}$ of the elements of $B$. I suspect we have something like:
$B = bigcup_{{x,y,z} text{ mentioned earlier}}bigcup_{a subset {x,y,z}} C_{{x,y,z}}(a)$
However, at this point I am stuck and I'm not sure how to proceed. I can't think of a nice way to count the size of $B$ and show it is what I want because I can't think of an easy way to account for all of the overlaps occurring.
combinatorics elementary-set-theory
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
$endgroup$
add a comment |
$begingroup$
Let $Asubset mathcal{P}({1, dots, n}) $ and $ B subset {1, dots, n }$
We say $A$ shatters $B$ if $forall y subset B, exists x in A$ such that $x cap B = y$.
I am asked to show that if $A$ does not shatter the sets: ${1,2,3},{2,3,4}, dots, {n-2,n-1,n,}, {n-1,n,1}, {n,1,2}$ and $n$ is a multiple of $3$ then $|A| leq 7^{frac{n}{3}}$
My current thinking is that, for each of these $3$-sets, we have to miss at least one of their subsets.
Specifically, for each $a subset {x,y,z}$ there are $2^{n-3}$ subsets of ${1,dots,n}$ that intersect with ${x,y,z}$ to give $a$. (Call the set of there $2^{n-3}$ subsets $C_{{x,y,z}}(a)$) Hence if $A$ does not shatter ${1,2,3}$ because we are missing $a$, then $A$ cannot contain these $2^{n-3}$ subsets.
I want to say that there is some subset $B subset mathcal{P}({1,dots,n}$ of size $8* 7^{frac{n}{3}}$ such that we must have $A subset B$, and we may only have at most $frac{7}{8}$ of the elements of $B$. I suspect we have something like:
$B = bigcup_{{x,y,z} text{ mentioned earlier}}bigcup_{a subset {x,y,z}} C_{{x,y,z}}(a)$
However, at this point I am stuck and I'm not sure how to proceed. I can't think of a nice way to count the size of $B$ and show it is what I want because I can't think of an easy way to account for all of the overlaps occurring.
combinatorics elementary-set-theory
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
$endgroup$
$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
2
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11
add a comment |
$begingroup$
Let $Asubset mathcal{P}({1, dots, n}) $ and $ B subset {1, dots, n }$
We say $A$ shatters $B$ if $forall y subset B, exists x in A$ such that $x cap B = y$.
I am asked to show that if $A$ does not shatter the sets: ${1,2,3},{2,3,4}, dots, {n-2,n-1,n,}, {n-1,n,1}, {n,1,2}$ and $n$ is a multiple of $3$ then $|A| leq 7^{frac{n}{3}}$
My current thinking is that, for each of these $3$-sets, we have to miss at least one of their subsets.
Specifically, for each $a subset {x,y,z}$ there are $2^{n-3}$ subsets of ${1,dots,n}$ that intersect with ${x,y,z}$ to give $a$. (Call the set of there $2^{n-3}$ subsets $C_{{x,y,z}}(a)$) Hence if $A$ does not shatter ${1,2,3}$ because we are missing $a$, then $A$ cannot contain these $2^{n-3}$ subsets.
I want to say that there is some subset $B subset mathcal{P}({1,dots,n}$ of size $8* 7^{frac{n}{3}}$ such that we must have $A subset B$, and we may only have at most $frac{7}{8}$ of the elements of $B$. I suspect we have something like:
$B = bigcup_{{x,y,z} text{ mentioned earlier}}bigcup_{a subset {x,y,z}} C_{{x,y,z}}(a)$
However, at this point I am stuck and I'm not sure how to proceed. I can't think of a nice way to count the size of $B$ and show it is what I want because I can't think of an easy way to account for all of the overlaps occurring.
combinatorics elementary-set-theory
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
$endgroup$
Let $Asubset mathcal{P}({1, dots, n}) $ and $ B subset {1, dots, n }$
We say $A$ shatters $B$ if $forall y subset B, exists x in A$ such that $x cap B = y$.
I am asked to show that if $A$ does not shatter the sets: ${1,2,3},{2,3,4}, dots, {n-2,n-1,n,}, {n-1,n,1}, {n,1,2}$ and $n$ is a multiple of $3$ then $|A| leq 7^{frac{n}{3}}$
My current thinking is that, for each of these $3$-sets, we have to miss at least one of their subsets.
Specifically, for each $a subset {x,y,z}$ there are $2^{n-3}$ subsets of ${1,dots,n}$ that intersect with ${x,y,z}$ to give $a$. (Call the set of there $2^{n-3}$ subsets $C_{{x,y,z}}(a)$) Hence if $A$ does not shatter ${1,2,3}$ because we are missing $a$, then $A$ cannot contain these $2^{n-3}$ subsets.
I want to say that there is some subset $B subset mathcal{P}({1,dots,n}$ of size $8* 7^{frac{n}{3}}$ such that we must have $A subset B$, and we may only have at most $frac{7}{8}$ of the elements of $B$. I suspect we have something like:
$B = bigcup_{{x,y,z} text{ mentioned earlier}}bigcup_{a subset {x,y,z}} C_{{x,y,z}}(a)$
However, at this point I am stuck and I'm not sure how to proceed. I can't think of a nice way to count the size of $B$ and show it is what I want because I can't think of an easy way to account for all of the overlaps occurring.
combinatorics elementary-set-theory
combinatorics elementary-set-theory
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
edited Dec 22 '18 at 15:10
user366818
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
asked Dec 20 '18 at 13:47
user366818user366818
1,056511
1,056511
$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
2
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11
add a comment |
$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
2
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11
$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
2
2
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11
add a comment |
1 Answer
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$begingroup$
Consider the $n/3$ sets
$$
{1,2,3},{4,5,6},ldots,{n-2,n-1,n}.
$$
Since $A$ doesn't shatter ${1,2,3}$, there exists some subset $T_{123}$ such that $S cap {1,2,3} neq T_{123}$ for all $S in A$. Define $T_{456},ldots$ similarly. Then
$$
A subseteq [mathcal{P}({1,2,3}) setminus {T_{123}}] times [mathcal{P}({4,5,6}) setminus {T_{456}}] times cdots times [mathcal{P}({n-2,n-1,n}) setminus {T_{n-2,n-1,n}}].
$$
Each of the $n/3$ factors on the right-hand side contains $2^3-1=7$ elements, and so the right-hand side consists of $7^{n/3}$ sets.
When $n > 3$, this bound isn't tight, since the right-hand side does shatter all other adjacent triplets.
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
$endgroup$
add a comment |
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$begingroup$
Consider the $n/3$ sets
$$
{1,2,3},{4,5,6},ldots,{n-2,n-1,n}.
$$
Since $A$ doesn't shatter ${1,2,3}$, there exists some subset $T_{123}$ such that $S cap {1,2,3} neq T_{123}$ for all $S in A$. Define $T_{456},ldots$ similarly. Then
$$
A subseteq [mathcal{P}({1,2,3}) setminus {T_{123}}] times [mathcal{P}({4,5,6}) setminus {T_{456}}] times cdots times [mathcal{P}({n-2,n-1,n}) setminus {T_{n-2,n-1,n}}].
$$
Each of the $n/3$ factors on the right-hand side contains $2^3-1=7$ elements, and so the right-hand side consists of $7^{n/3}$ sets.
When $n > 3$, this bound isn't tight, since the right-hand side does shatter all other adjacent triplets.
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
$endgroup$
add a comment |
$begingroup$
Consider the $n/3$ sets
$$
{1,2,3},{4,5,6},ldots,{n-2,n-1,n}.
$$
Since $A$ doesn't shatter ${1,2,3}$, there exists some subset $T_{123}$ such that $S cap {1,2,3} neq T_{123}$ for all $S in A$. Define $T_{456},ldots$ similarly. Then
$$
A subseteq [mathcal{P}({1,2,3}) setminus {T_{123}}] times [mathcal{P}({4,5,6}) setminus {T_{456}}] times cdots times [mathcal{P}({n-2,n-1,n}) setminus {T_{n-2,n-1,n}}].
$$
Each of the $n/3$ factors on the right-hand side contains $2^3-1=7$ elements, and so the right-hand side consists of $7^{n/3}$ sets.
When $n > 3$, this bound isn't tight, since the right-hand side does shatter all other adjacent triplets.
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
$endgroup$
add a comment |
$begingroup$
Consider the $n/3$ sets
$$
{1,2,3},{4,5,6},ldots,{n-2,n-1,n}.
$$
Since $A$ doesn't shatter ${1,2,3}$, there exists some subset $T_{123}$ such that $S cap {1,2,3} neq T_{123}$ for all $S in A$. Define $T_{456},ldots$ similarly. Then
$$
A subseteq [mathcal{P}({1,2,3}) setminus {T_{123}}] times [mathcal{P}({4,5,6}) setminus {T_{456}}] times cdots times [mathcal{P}({n-2,n-1,n}) setminus {T_{n-2,n-1,n}}].
$$
Each of the $n/3$ factors on the right-hand side contains $2^3-1=7$ elements, and so the right-hand side consists of $7^{n/3}$ sets.
When $n > 3$, this bound isn't tight, since the right-hand side does shatter all other adjacent triplets.
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
$endgroup$
Consider the $n/3$ sets
$$
{1,2,3},{4,5,6},ldots,{n-2,n-1,n}.
$$
Since $A$ doesn't shatter ${1,2,3}$, there exists some subset $T_{123}$ such that $S cap {1,2,3} neq T_{123}$ for all $S in A$. Define $T_{456},ldots$ similarly. Then
$$
A subseteq [mathcal{P}({1,2,3}) setminus {T_{123}}] times [mathcal{P}({4,5,6}) setminus {T_{456}}] times cdots times [mathcal{P}({n-2,n-1,n}) setminus {T_{n-2,n-1,n}}].
$$
Each of the $n/3$ factors on the right-hand side contains $2^3-1=7$ elements, and so the right-hand side consists of $7^{n/3}$ sets.
When $n > 3$, this bound isn't tight, since the right-hand side does shatter all other adjacent triplets.
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
answered Dec 22 '18 at 16:09
Yuval FilmusYuval Filmus
49k472148
49k472148
add a comment |
add a comment |
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$begingroup$
Since $mathcal{P}({1, dots, n})$ is a family of (non-empty) subsets of ${1, dots, n}$ and $A$, $B$ are subsets of this family, they are also families of sets. Then $A$ does not shatter the sets ... means $A$ does not shatter the family consisting of these sets, right?
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:43
2
$begingroup$
Anyway, I don’t understand the definition of $A$ shatters $B$. If $A$ shatters $B$ then, $Bsubset B$ so there exists $xsubset A$ such that $xcap B=B$, that is $xsupset B$. So $Asupset B$. Conversely, if $Asupset B$ than for any $ysubset B$ we have $ysubset A$ so if we put $x=y$ then $xcap B=y$. That is $A$ shatters $B$ iff $Asupset B$.
$endgroup$
– Alex Ravsky
Dec 21 '18 at 3:44
$begingroup$
@AlexRavsky my apologies, I wrote the definition wrong. Firstly, $A$ is a family of subsets and $B$ is a subset. Secondly, $A$ shatters $B$ if for all $y subset B ; exists x in A$ such that $x cap B = y$
$endgroup$
– user366818
Dec 22 '18 at 15:11