Combinatorics- distribute n objects to k bin with constraints
$begingroup$
Suppose I have n objects distributed to k bins and I want to know how many multisets satisfy the following condition: exactly j bins have at least m objects, where j $leq$ k,m $leq$ n, and j*m $leq$ n. For example, if n = 5, k=4, j=2 and m = 2, there are 24 subsets:
[3, 2, 0, 0]
[3, 0, 2, 0]
[3, 0, 0, 2]
[2, 3, 0, 0]
[2, 2, 1, 0]
[2, 2, 0, 1]
[2, 1, 2, 0]
[2, 1, 0, 2]
[2, 0, 3, 0]
[2, 0, 2, 1]
[2, 0, 1, 2]
[2, 0, 0, 3]
[1, 2, 2, 0]
[1, 2, 0, 2]
[1, 0, 2, 2]
[0, 3, 2, 0]
[0, 3, 0, 2]
[0, 2, 3, 0]
[0, 2, 2, 1]
[0, 2, 1, 2]
[0, 2, 0, 3]
[0, 1, 2, 2]
[0, 0, 3, 2]
[0, 0, 2, 3]
I'm having difficulty finding a general formula for this problem. Any help would be appreciated.
combinatorics
$endgroup$
add a comment |
$begingroup$
Suppose I have n objects distributed to k bins and I want to know how many multisets satisfy the following condition: exactly j bins have at least m objects, where j $leq$ k,m $leq$ n, and j*m $leq$ n. For example, if n = 5, k=4, j=2 and m = 2, there are 24 subsets:
[3, 2, 0, 0]
[3, 0, 2, 0]
[3, 0, 0, 2]
[2, 3, 0, 0]
[2, 2, 1, 0]
[2, 2, 0, 1]
[2, 1, 2, 0]
[2, 1, 0, 2]
[2, 0, 3, 0]
[2, 0, 2, 1]
[2, 0, 1, 2]
[2, 0, 0, 3]
[1, 2, 2, 0]
[1, 2, 0, 2]
[1, 0, 2, 2]
[0, 3, 2, 0]
[0, 3, 0, 2]
[0, 2, 3, 0]
[0, 2, 2, 1]
[0, 2, 1, 2]
[0, 2, 0, 3]
[0, 1, 2, 2]
[0, 0, 3, 2]
[0, 0, 2, 3]
I'm having difficulty finding a general formula for this problem. Any help would be appreciated.
combinatorics
$endgroup$
add a comment |
$begingroup$
Suppose I have n objects distributed to k bins and I want to know how many multisets satisfy the following condition: exactly j bins have at least m objects, where j $leq$ k,m $leq$ n, and j*m $leq$ n. For example, if n = 5, k=4, j=2 and m = 2, there are 24 subsets:
[3, 2, 0, 0]
[3, 0, 2, 0]
[3, 0, 0, 2]
[2, 3, 0, 0]
[2, 2, 1, 0]
[2, 2, 0, 1]
[2, 1, 2, 0]
[2, 1, 0, 2]
[2, 0, 3, 0]
[2, 0, 2, 1]
[2, 0, 1, 2]
[2, 0, 0, 3]
[1, 2, 2, 0]
[1, 2, 0, 2]
[1, 0, 2, 2]
[0, 3, 2, 0]
[0, 3, 0, 2]
[0, 2, 3, 0]
[0, 2, 2, 1]
[0, 2, 1, 2]
[0, 2, 0, 3]
[0, 1, 2, 2]
[0, 0, 3, 2]
[0, 0, 2, 3]
I'm having difficulty finding a general formula for this problem. Any help would be appreciated.
combinatorics
$endgroup$
Suppose I have n objects distributed to k bins and I want to know how many multisets satisfy the following condition: exactly j bins have at least m objects, where j $leq$ k,m $leq$ n, and j*m $leq$ n. For example, if n = 5, k=4, j=2 and m = 2, there are 24 subsets:
[3, 2, 0, 0]
[3, 0, 2, 0]
[3, 0, 0, 2]
[2, 3, 0, 0]
[2, 2, 1, 0]
[2, 2, 0, 1]
[2, 1, 2, 0]
[2, 1, 0, 2]
[2, 0, 3, 0]
[2, 0, 2, 1]
[2, 0, 1, 2]
[2, 0, 0, 3]
[1, 2, 2, 0]
[1, 2, 0, 2]
[1, 0, 2, 2]
[0, 3, 2, 0]
[0, 3, 0, 2]
[0, 2, 3, 0]
[0, 2, 2, 1]
[0, 2, 1, 2]
[0, 2, 0, 3]
[0, 1, 2, 2]
[0, 0, 3, 2]
[0, 0, 2, 3]
I'm having difficulty finding a general formula for this problem. Any help would be appreciated.
combinatorics
combinatorics
asked Jan 7 at 17:32
Christopher FisherChristopher Fisher
32
32
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Choose which of the $j$ bins have at least $m$ objects in $binom{k}j$ ways. Let $x_1,x_2,dots,x_j$ be number of objects minus $j$ in these $j$ bins. Let $y_1,y_2,dots,y_{k-j}$ be the number of object in the other bins. Then
$$
begin{align}
x_1+dots+x_j+y_1+dots+y_{k-j}&=n-jm,\
0 &le x_i,\
0&le y_i<m
end{align}
$$
If we ignored the condition $y_i<m$, then we would have several non negative integers summing to $n-jm$; the number of ways to do this is $binom{n-jm+k-1}{k-1}$, using stars and bars. In order to account for $y_i<m$, we use inclusion exclusion. Namely, subtract out the bad solutions where $y_ige m$ for each $i$, then add back in the doubly subtracted solutions where $y_ige m$ and $y_jge m$, then subtract out the triply bad solutions, etc. The result is
$$
binom{k}jsum_{ell = 0}^{k-j}binom{k-j}{ell}(-1)^ell binom{n-(j+ell)m+k-1}{k-1}
$$
Here, I am using the convention that $binom{n}k=0$ when $n<k$.
$endgroup$
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
add a comment |
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1 Answer
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1 Answer
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oldest
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$begingroup$
Choose which of the $j$ bins have at least $m$ objects in $binom{k}j$ ways. Let $x_1,x_2,dots,x_j$ be number of objects minus $j$ in these $j$ bins. Let $y_1,y_2,dots,y_{k-j}$ be the number of object in the other bins. Then
$$
begin{align}
x_1+dots+x_j+y_1+dots+y_{k-j}&=n-jm,\
0 &le x_i,\
0&le y_i<m
end{align}
$$
If we ignored the condition $y_i<m$, then we would have several non negative integers summing to $n-jm$; the number of ways to do this is $binom{n-jm+k-1}{k-1}$, using stars and bars. In order to account for $y_i<m$, we use inclusion exclusion. Namely, subtract out the bad solutions where $y_ige m$ for each $i$, then add back in the doubly subtracted solutions where $y_ige m$ and $y_jge m$, then subtract out the triply bad solutions, etc. The result is
$$
binom{k}jsum_{ell = 0}^{k-j}binom{k-j}{ell}(-1)^ell binom{n-(j+ell)m+k-1}{k-1}
$$
Here, I am using the convention that $binom{n}k=0$ when $n<k$.
$endgroup$
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
add a comment |
$begingroup$
Choose which of the $j$ bins have at least $m$ objects in $binom{k}j$ ways. Let $x_1,x_2,dots,x_j$ be number of objects minus $j$ in these $j$ bins. Let $y_1,y_2,dots,y_{k-j}$ be the number of object in the other bins. Then
$$
begin{align}
x_1+dots+x_j+y_1+dots+y_{k-j}&=n-jm,\
0 &le x_i,\
0&le y_i<m
end{align}
$$
If we ignored the condition $y_i<m$, then we would have several non negative integers summing to $n-jm$; the number of ways to do this is $binom{n-jm+k-1}{k-1}$, using stars and bars. In order to account for $y_i<m$, we use inclusion exclusion. Namely, subtract out the bad solutions where $y_ige m$ for each $i$, then add back in the doubly subtracted solutions where $y_ige m$ and $y_jge m$, then subtract out the triply bad solutions, etc. The result is
$$
binom{k}jsum_{ell = 0}^{k-j}binom{k-j}{ell}(-1)^ell binom{n-(j+ell)m+k-1}{k-1}
$$
Here, I am using the convention that $binom{n}k=0$ when $n<k$.
$endgroup$
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
add a comment |
$begingroup$
Choose which of the $j$ bins have at least $m$ objects in $binom{k}j$ ways. Let $x_1,x_2,dots,x_j$ be number of objects minus $j$ in these $j$ bins. Let $y_1,y_2,dots,y_{k-j}$ be the number of object in the other bins. Then
$$
begin{align}
x_1+dots+x_j+y_1+dots+y_{k-j}&=n-jm,\
0 &le x_i,\
0&le y_i<m
end{align}
$$
If we ignored the condition $y_i<m$, then we would have several non negative integers summing to $n-jm$; the number of ways to do this is $binom{n-jm+k-1}{k-1}$, using stars and bars. In order to account for $y_i<m$, we use inclusion exclusion. Namely, subtract out the bad solutions where $y_ige m$ for each $i$, then add back in the doubly subtracted solutions where $y_ige m$ and $y_jge m$, then subtract out the triply bad solutions, etc. The result is
$$
binom{k}jsum_{ell = 0}^{k-j}binom{k-j}{ell}(-1)^ell binom{n-(j+ell)m+k-1}{k-1}
$$
Here, I am using the convention that $binom{n}k=0$ when $n<k$.
$endgroup$
Choose which of the $j$ bins have at least $m$ objects in $binom{k}j$ ways. Let $x_1,x_2,dots,x_j$ be number of objects minus $j$ in these $j$ bins. Let $y_1,y_2,dots,y_{k-j}$ be the number of object in the other bins. Then
$$
begin{align}
x_1+dots+x_j+y_1+dots+y_{k-j}&=n-jm,\
0 &le x_i,\
0&le y_i<m
end{align}
$$
If we ignored the condition $y_i<m$, then we would have several non negative integers summing to $n-jm$; the number of ways to do this is $binom{n-jm+k-1}{k-1}$, using stars and bars. In order to account for $y_i<m$, we use inclusion exclusion. Namely, subtract out the bad solutions where $y_ige m$ for each $i$, then add back in the doubly subtracted solutions where $y_ige m$ and $y_jge m$, then subtract out the triply bad solutions, etc. The result is
$$
binom{k}jsum_{ell = 0}^{k-j}binom{k-j}{ell}(-1)^ell binom{n-(j+ell)m+k-1}{k-1}
$$
Here, I am using the convention that $binom{n}k=0$ when $n<k$.
answered Jan 7 at 18:46
Mike EarnestMike Earnest
23.3k12051
23.3k12051
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
add a comment |
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
Thank you for your help. I believe I identified a case where it fails: n=4,k=3,m=4,j=1. I think it should equal ${k choose j}$ = 3. I suspect the inclusion-exclusion method might be failing because something is overcounted in certain cases.
$endgroup$
– Christopher Fisher
Jan 7 at 20:40
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
If you apply my formula in your comment, you should get $$binom{3}1left(binom{2}0binom{2}2-binom{2}1binom{-2}{2}+binom{2}2binom{-6}2right)=3(1-0+0)=3,$$ which is correct.
$endgroup$
– Mike Earnest
Jan 7 at 21:10
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
You are right. Sorry for the confusion. The program I am using yielded ${-2 choose 1}$ = -2, which was unexpected. Thanks again!
$endgroup$
– Christopher Fisher
Jan 7 at 21:23
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
$begingroup$
The confusion is my fault. Most mathematicians agree that $binom{-2}1=-2$; it just makes my formula more convenient to write if you assume $binom{-2}1=0$.
$endgroup$
– Mike Earnest
Jan 7 at 22:27
add a comment |
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