In how many ways can I write $0$ as a sum of $n; 0s, 1s ;text{and}; -1s?$
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In how many ways can I write $0$ as a sum of $n; 0s, 1s ;text{or}; -1s?$ (Taking the order into account).
I suspect there is no closed formula to express the result, but I'd like someone to confirm it, or deny it.
Edit:
e.g., if $n=3$
$$begin{aligned}0&=;;;0+0+0,\ 0&=;;;1-1+0,\ 0&=;;;1+0-1,\ 0&=;;;0+1-1,\ 0&=-1+1+0,\ 0&=-1+0+1,\ 0&=;;;0-1+1.end{aligned}$$
So for $n=3;$ there are $7$ ways.
combinatorics
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|
show 1 more comment
$begingroup$
In how many ways can I write $0$ as a sum of $n; 0s, 1s ;text{or}; -1s?$ (Taking the order into account).
I suspect there is no closed formula to express the result, but I'd like someone to confirm it, or deny it.
Edit:
e.g., if $n=3$
$$begin{aligned}0&=;;;0+0+0,\ 0&=;;;1-1+0,\ 0&=;;;1+0-1,\ 0&=;;;0+1-1,\ 0&=-1+1+0,\ 0&=-1+0+1,\ 0&=;;;0-1+1.end{aligned}$$
So for $n=3;$ there are $7$ ways.
combinatorics
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1
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Set up a recurrence perhaps.
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– paw88789
Dec 29 '18 at 16:35
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do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
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Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
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@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
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– timtfj
Dec 29 '18 at 17:11
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I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25
|
show 1 more comment
$begingroup$
In how many ways can I write $0$ as a sum of $n; 0s, 1s ;text{or}; -1s?$ (Taking the order into account).
I suspect there is no closed formula to express the result, but I'd like someone to confirm it, or deny it.
Edit:
e.g., if $n=3$
$$begin{aligned}0&=;;;0+0+0,\ 0&=;;;1-1+0,\ 0&=;;;1+0-1,\ 0&=;;;0+1-1,\ 0&=-1+1+0,\ 0&=-1+0+1,\ 0&=;;;0-1+1.end{aligned}$$
So for $n=3;$ there are $7$ ways.
combinatorics
$endgroup$
In how many ways can I write $0$ as a sum of $n; 0s, 1s ;text{or}; -1s?$ (Taking the order into account).
I suspect there is no closed formula to express the result, but I'd like someone to confirm it, or deny it.
Edit:
e.g., if $n=3$
$$begin{aligned}0&=;;;0+0+0,\ 0&=;;;1-1+0,\ 0&=;;;1+0-1,\ 0&=;;;0+1-1,\ 0&=-1+1+0,\ 0&=-1+0+1,\ 0&=;;;0-1+1.end{aligned}$$
So for $n=3;$ there are $7$ ways.
combinatorics
combinatorics
edited Dec 29 '18 at 22:01
user376343
3,9834829
3,9834829
asked Dec 29 '18 at 16:31
Lucio TanziniLucio Tanzini
368114
368114
1
$begingroup$
Set up a recurrence perhaps.
$endgroup$
– paw88789
Dec 29 '18 at 16:35
$begingroup$
do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
$begingroup$
Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
$begingroup$
@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
$endgroup$
– timtfj
Dec 29 '18 at 17:11
$begingroup$
I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25
|
show 1 more comment
1
$begingroup$
Set up a recurrence perhaps.
$endgroup$
– paw88789
Dec 29 '18 at 16:35
$begingroup$
do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
$begingroup$
Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
$begingroup$
@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
$endgroup$
– timtfj
Dec 29 '18 at 17:11
$begingroup$
I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25
1
1
$begingroup$
Set up a recurrence perhaps.
$endgroup$
– paw88789
Dec 29 '18 at 16:35
$begingroup$
Set up a recurrence perhaps.
$endgroup$
– paw88789
Dec 29 '18 at 16:35
$begingroup$
do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
$begingroup$
do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
$begingroup$
Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
$begingroup$
Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
$begingroup$
@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
$endgroup$
– timtfj
Dec 29 '18 at 17:11
$begingroup$
@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
$endgroup$
– timtfj
Dec 29 '18 at 17:11
$begingroup$
I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25
$begingroup$
I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25
|
show 1 more comment
2 Answers
2
active
oldest
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$begingroup$
$s(n) = sum_{k=0}^{n/2} binom{n}{n-2k} cdot binom{2k}{k}$ The first term is the number of ways to arrange the zeroes, and then the second term arranges the parities. Now, we can simplify further:
$s(n) = sum_{k=0}^{n/2} frac{2k!}{2k!} cdot frac{n!}{(n-2k)!(k!)(k!)} = sum_{k=0}^{n/2} frac{n!}{(n-2k)!(k!)(k!)}$
Edit: put a few values into the OIES and came across trinomial coeffecients. In particular, s(n) is the n-th central trinomial coefficient, which has several closed forms, which you can find in the second link.
$endgroup$
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
add a comment |
$begingroup$
The number of $0s, 1s$ and $-1s$ possible for a particular $n$ can be seen by the number of solutions to:
$$2p+q=n| p,q in Bbb Z^+$$
This can be done in $frac{n+1}{2}$ ways for odd $n$ and $frac{n+2}{2}$ ways for even $n$
You'll just need to account for positioning after this.
$endgroup$
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
add a comment |
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2 Answers
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2 Answers
2
active
oldest
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$begingroup$
$s(n) = sum_{k=0}^{n/2} binom{n}{n-2k} cdot binom{2k}{k}$ The first term is the number of ways to arrange the zeroes, and then the second term arranges the parities. Now, we can simplify further:
$s(n) = sum_{k=0}^{n/2} frac{2k!}{2k!} cdot frac{n!}{(n-2k)!(k!)(k!)} = sum_{k=0}^{n/2} frac{n!}{(n-2k)!(k!)(k!)}$
Edit: put a few values into the OIES and came across trinomial coeffecients. In particular, s(n) is the n-th central trinomial coefficient, which has several closed forms, which you can find in the second link.
$endgroup$
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
add a comment |
$begingroup$
$s(n) = sum_{k=0}^{n/2} binom{n}{n-2k} cdot binom{2k}{k}$ The first term is the number of ways to arrange the zeroes, and then the second term arranges the parities. Now, we can simplify further:
$s(n) = sum_{k=0}^{n/2} frac{2k!}{2k!} cdot frac{n!}{(n-2k)!(k!)(k!)} = sum_{k=0}^{n/2} frac{n!}{(n-2k)!(k!)(k!)}$
Edit: put a few values into the OIES and came across trinomial coeffecients. In particular, s(n) is the n-th central trinomial coefficient, which has several closed forms, which you can find in the second link.
$endgroup$
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
add a comment |
$begingroup$
$s(n) = sum_{k=0}^{n/2} binom{n}{n-2k} cdot binom{2k}{k}$ The first term is the number of ways to arrange the zeroes, and then the second term arranges the parities. Now, we can simplify further:
$s(n) = sum_{k=0}^{n/2} frac{2k!}{2k!} cdot frac{n!}{(n-2k)!(k!)(k!)} = sum_{k=0}^{n/2} frac{n!}{(n-2k)!(k!)(k!)}$
Edit: put a few values into the OIES and came across trinomial coeffecients. In particular, s(n) is the n-th central trinomial coefficient, which has several closed forms, which you can find in the second link.
$endgroup$
$s(n) = sum_{k=0}^{n/2} binom{n}{n-2k} cdot binom{2k}{k}$ The first term is the number of ways to arrange the zeroes, and then the second term arranges the parities. Now, we can simplify further:
$s(n) = sum_{k=0}^{n/2} frac{2k!}{2k!} cdot frac{n!}{(n-2k)!(k!)(k!)} = sum_{k=0}^{n/2} frac{n!}{(n-2k)!(k!)(k!)}$
Edit: put a few values into the OIES and came across trinomial coeffecients. In particular, s(n) is the n-th central trinomial coefficient, which has several closed forms, which you can find in the second link.
edited Dec 29 '18 at 21:48
answered Dec 29 '18 at 18:53
Zachary HunterZachary Hunter
1,065314
1,065314
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
add a comment |
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
This looks wrong to me. There are $k$ $1$'s, $k$ $(-1)$'s and $n-2k$ zeroes, so the denominator should be $(k!)^2(n-2k)!$.
$endgroup$
– timtfj
Dec 29 '18 at 19:07
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Whoops yeah I forgot a term
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:12
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
Yes, I got the same formula. Do you think It can be simplified into a closed formula?
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 19:31
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
@LucioTanzini nothing immediately comes to mind without getting my hands dirty, but I’m thinking on it.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 19:37
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
$begingroup$
Update: found closed form.
$endgroup$
– Zachary Hunter
Dec 29 '18 at 20:43
add a comment |
$begingroup$
The number of $0s, 1s$ and $-1s$ possible for a particular $n$ can be seen by the number of solutions to:
$$2p+q=n| p,q in Bbb Z^+$$
This can be done in $frac{n+1}{2}$ ways for odd $n$ and $frac{n+2}{2}$ ways for even $n$
You'll just need to account for positioning after this.
$endgroup$
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
add a comment |
$begingroup$
The number of $0s, 1s$ and $-1s$ possible for a particular $n$ can be seen by the number of solutions to:
$$2p+q=n| p,q in Bbb Z^+$$
This can be done in $frac{n+1}{2}$ ways for odd $n$ and $frac{n+2}{2}$ ways for even $n$
You'll just need to account for positioning after this.
$endgroup$
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
add a comment |
$begingroup$
The number of $0s, 1s$ and $-1s$ possible for a particular $n$ can be seen by the number of solutions to:
$$2p+q=n| p,q in Bbb Z^+$$
This can be done in $frac{n+1}{2}$ ways for odd $n$ and $frac{n+2}{2}$ ways for even $n$
You'll just need to account for positioning after this.
$endgroup$
The number of $0s, 1s$ and $-1s$ possible for a particular $n$ can be seen by the number of solutions to:
$$2p+q=n| p,q in Bbb Z^+$$
This can be done in $frac{n+1}{2}$ ways for odd $n$ and $frac{n+2}{2}$ ways for even $n$
You'll just need to account for positioning after this.
edited Dec 29 '18 at 17:16
answered Dec 29 '18 at 16:55
Rhys HughesRhys Hughes
7,1051630
7,1051630
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
add a comment |
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
$begingroup$
Yes, but I'm afraid positioning is the main problem Indeed
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 17:34
add a comment |
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1
$begingroup$
Set up a recurrence perhaps.
$endgroup$
– paw88789
Dec 29 '18 at 16:35
$begingroup$
do you mean n each or a total of n?
$endgroup$
– player100
Dec 29 '18 at 16:45
$begingroup$
Sorry I meant that in total, the number of 0's, 1's and -1's is n
$endgroup$
– Lucio Tanzini
Dec 29 '18 at 16:59
$begingroup$
@MJD Thsnks for pointing out that I'd missed the $n$. I've deleted the answer!
$endgroup$
– timtfj
Dec 29 '18 at 17:11
$begingroup$
I'd start here: for each $n$, we can choose how many times $0$ occurs ,and how many times $1$ occurs. $-1$ then has to match $1$. So if $0$ is used $r$ times and $1$ is used $s$ times, $r=n-2s$.Then for each choice of $r$ and $s$ you've got a defined permutation problem, with $r, s$ and $s$ items..
$endgroup$
– timtfj
Dec 29 '18 at 17:25