Sum of freely chosen subset of $n$-tuple is divisible by $n$.
$begingroup$
I am struggling with the following task:
Be $n in N$ and $(a_1, a_2, ldots, a_n) in mathbb{Z}^n$. Prove that there is always $i, j in underline{n}$, with $i≤j$, so that $sum_{k=i}^j a_k$ is divisible by n.
My first idea was to use a distinction of cases. In the first case a multiple of n is an element of $mathbb{Z}^n$ so you could select the sum to be just this element so it would obviously be divisible.
In the second case, all elements of the tuple are the same, so that adding them all up will result in a multiple of $n$ once more, making it divisible.
I am lost however on what to do in the third case, where the two above are not true. I would truly appreciate your input.
elementary-number-theory discrete-mathematics divisibility
$endgroup$
add a comment |
$begingroup$
I am struggling with the following task:
Be $n in N$ and $(a_1, a_2, ldots, a_n) in mathbb{Z}^n$. Prove that there is always $i, j in underline{n}$, with $i≤j$, so that $sum_{k=i}^j a_k$ is divisible by n.
My first idea was to use a distinction of cases. In the first case a multiple of n is an element of $mathbb{Z}^n$ so you could select the sum to be just this element so it would obviously be divisible.
In the second case, all elements of the tuple are the same, so that adding them all up will result in a multiple of $n$ once more, making it divisible.
I am lost however on what to do in the third case, where the two above are not true. I would truly appreciate your input.
elementary-number-theory discrete-mathematics divisibility
$endgroup$
add a comment |
$begingroup$
I am struggling with the following task:
Be $n in N$ and $(a_1, a_2, ldots, a_n) in mathbb{Z}^n$. Prove that there is always $i, j in underline{n}$, with $i≤j$, so that $sum_{k=i}^j a_k$ is divisible by n.
My first idea was to use a distinction of cases. In the first case a multiple of n is an element of $mathbb{Z}^n$ so you could select the sum to be just this element so it would obviously be divisible.
In the second case, all elements of the tuple are the same, so that adding them all up will result in a multiple of $n$ once more, making it divisible.
I am lost however on what to do in the third case, where the two above are not true. I would truly appreciate your input.
elementary-number-theory discrete-mathematics divisibility
$endgroup$
I am struggling with the following task:
Be $n in N$ and $(a_1, a_2, ldots, a_n) in mathbb{Z}^n$. Prove that there is always $i, j in underline{n}$, with $i≤j$, so that $sum_{k=i}^j a_k$ is divisible by n.
My first idea was to use a distinction of cases. In the first case a multiple of n is an element of $mathbb{Z}^n$ so you could select the sum to be just this element so it would obviously be divisible.
In the second case, all elements of the tuple are the same, so that adding them all up will result in a multiple of $n$ once more, making it divisible.
I am lost however on what to do in the third case, where the two above are not true. I would truly appreciate your input.
elementary-number-theory discrete-mathematics divisibility
elementary-number-theory discrete-mathematics divisibility
edited Dec 13 '18 at 14:12
rtybase
10.9k21533
10.9k21533
asked Dec 13 '18 at 12:11
JulianGiJulianGi
111
111
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Some elements of $(a_1, a_2, ldots, a_n)$ may be different and it need not contain a multiple of $n$ of course. (it is chosen just randomly from $mathbb{Z}^n$.)
You need a little trick to solve this problem in an easy way. Consider $n$ values
$$
a_1, a_1+a_2, a_1+a_2+a_3, ldots , a_1+a_2 +cdots a_n.
$$ If at least one of them is a multiple of $n$, it's okay. If not, what can be said about their remainders divided by $n$?
$endgroup$
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
add a comment |
$begingroup$
Using pigeonhole principle, let's look at all the remainders of
$$a_1 pmod{n}$$
$$a_1+a_2pmod{n}$$
$$a_1+a_2+a_3pmod{n}$$
$$...$$
$$a_1+a_2+a_3+...+a_npmod{n}$$
If at least one is $0$, we are done.
If none is $0$, then at least $2$ will have the same value (there will be $n$ remainders, taking $n-1$ possible values within ${1,...,n-1}$, pigeonhole principle). E.g.
$$a_1+a_2+a_3+...+a_j equiv a_1+a_2+a_3+...+a_i pmod{n}$$
The remaining part is to subtract, assume $i<j$
$$(a_1+a_2+a_3+...+a_j)- (a_1+a_2+a_3+...+a_i)equiv 0 pmod{n}$$
or
$$a_{j+1}+a_{j+2}+a_{j+3}+...+a_iequiv 0 pmod{n}$$
$endgroup$
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
add a comment |
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2 Answers
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2 Answers
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$begingroup$
Some elements of $(a_1, a_2, ldots, a_n)$ may be different and it need not contain a multiple of $n$ of course. (it is chosen just randomly from $mathbb{Z}^n$.)
You need a little trick to solve this problem in an easy way. Consider $n$ values
$$
a_1, a_1+a_2, a_1+a_2+a_3, ldots , a_1+a_2 +cdots a_n.
$$ If at least one of them is a multiple of $n$, it's okay. If not, what can be said about their remainders divided by $n$?
$endgroup$
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
add a comment |
$begingroup$
Some elements of $(a_1, a_2, ldots, a_n)$ may be different and it need not contain a multiple of $n$ of course. (it is chosen just randomly from $mathbb{Z}^n$.)
You need a little trick to solve this problem in an easy way. Consider $n$ values
$$
a_1, a_1+a_2, a_1+a_2+a_3, ldots , a_1+a_2 +cdots a_n.
$$ If at least one of them is a multiple of $n$, it's okay. If not, what can be said about their remainders divided by $n$?
$endgroup$
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
add a comment |
$begingroup$
Some elements of $(a_1, a_2, ldots, a_n)$ may be different and it need not contain a multiple of $n$ of course. (it is chosen just randomly from $mathbb{Z}^n$.)
You need a little trick to solve this problem in an easy way. Consider $n$ values
$$
a_1, a_1+a_2, a_1+a_2+a_3, ldots , a_1+a_2 +cdots a_n.
$$ If at least one of them is a multiple of $n$, it's okay. If not, what can be said about their remainders divided by $n$?
$endgroup$
Some elements of $(a_1, a_2, ldots, a_n)$ may be different and it need not contain a multiple of $n$ of course. (it is chosen just randomly from $mathbb{Z}^n$.)
You need a little trick to solve this problem in an easy way. Consider $n$ values
$$
a_1, a_1+a_2, a_1+a_2+a_3, ldots , a_1+a_2 +cdots a_n.
$$ If at least one of them is a multiple of $n$, it's okay. If not, what can be said about their remainders divided by $n$?
edited Dec 13 '18 at 12:24
answered Dec 13 '18 at 12:16
SongSong
12.2k630
12.2k630
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
add a comment |
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
1
1
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
$begingroup$
I think we have almost identical answers (+1).
$endgroup$
– rtybase
Dec 13 '18 at 12:31
add a comment |
$begingroup$
Using pigeonhole principle, let's look at all the remainders of
$$a_1 pmod{n}$$
$$a_1+a_2pmod{n}$$
$$a_1+a_2+a_3pmod{n}$$
$$...$$
$$a_1+a_2+a_3+...+a_npmod{n}$$
If at least one is $0$, we are done.
If none is $0$, then at least $2$ will have the same value (there will be $n$ remainders, taking $n-1$ possible values within ${1,...,n-1}$, pigeonhole principle). E.g.
$$a_1+a_2+a_3+...+a_j equiv a_1+a_2+a_3+...+a_i pmod{n}$$
The remaining part is to subtract, assume $i<j$
$$(a_1+a_2+a_3+...+a_j)- (a_1+a_2+a_3+...+a_i)equiv 0 pmod{n}$$
or
$$a_{j+1}+a_{j+2}+a_{j+3}+...+a_iequiv 0 pmod{n}$$
$endgroup$
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
add a comment |
$begingroup$
Using pigeonhole principle, let's look at all the remainders of
$$a_1 pmod{n}$$
$$a_1+a_2pmod{n}$$
$$a_1+a_2+a_3pmod{n}$$
$$...$$
$$a_1+a_2+a_3+...+a_npmod{n}$$
If at least one is $0$, we are done.
If none is $0$, then at least $2$ will have the same value (there will be $n$ remainders, taking $n-1$ possible values within ${1,...,n-1}$, pigeonhole principle). E.g.
$$a_1+a_2+a_3+...+a_j equiv a_1+a_2+a_3+...+a_i pmod{n}$$
The remaining part is to subtract, assume $i<j$
$$(a_1+a_2+a_3+...+a_j)- (a_1+a_2+a_3+...+a_i)equiv 0 pmod{n}$$
or
$$a_{j+1}+a_{j+2}+a_{j+3}+...+a_iequiv 0 pmod{n}$$
$endgroup$
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
add a comment |
$begingroup$
Using pigeonhole principle, let's look at all the remainders of
$$a_1 pmod{n}$$
$$a_1+a_2pmod{n}$$
$$a_1+a_2+a_3pmod{n}$$
$$...$$
$$a_1+a_2+a_3+...+a_npmod{n}$$
If at least one is $0$, we are done.
If none is $0$, then at least $2$ will have the same value (there will be $n$ remainders, taking $n-1$ possible values within ${1,...,n-1}$, pigeonhole principle). E.g.
$$a_1+a_2+a_3+...+a_j equiv a_1+a_2+a_3+...+a_i pmod{n}$$
The remaining part is to subtract, assume $i<j$
$$(a_1+a_2+a_3+...+a_j)- (a_1+a_2+a_3+...+a_i)equiv 0 pmod{n}$$
or
$$a_{j+1}+a_{j+2}+a_{j+3}+...+a_iequiv 0 pmod{n}$$
$endgroup$
Using pigeonhole principle, let's look at all the remainders of
$$a_1 pmod{n}$$
$$a_1+a_2pmod{n}$$
$$a_1+a_2+a_3pmod{n}$$
$$...$$
$$a_1+a_2+a_3+...+a_npmod{n}$$
If at least one is $0$, we are done.
If none is $0$, then at least $2$ will have the same value (there will be $n$ remainders, taking $n-1$ possible values within ${1,...,n-1}$, pigeonhole principle). E.g.
$$a_1+a_2+a_3+...+a_j equiv a_1+a_2+a_3+...+a_i pmod{n}$$
The remaining part is to subtract, assume $i<j$
$$(a_1+a_2+a_3+...+a_j)- (a_1+a_2+a_3+...+a_i)equiv 0 pmod{n}$$
or
$$a_{j+1}+a_{j+2}+a_{j+3}+...+a_iequiv 0 pmod{n}$$
edited Dec 13 '18 at 14:07
answered Dec 13 '18 at 12:21
rtybasertybase
10.9k21533
10.9k21533
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
add a comment |
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
1
1
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
This is very extensive and well written. Thanks!
$endgroup$
– Empty2k12
Dec 13 '18 at 12:59
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
$begingroup$
Thanks for your answer. Now I understand how this works. However I have trouble to understand how you reach the conclusion, that "If none is 0, then at least 2 will have the same value". If this would be given the rest of your calculations fall into place.
$endgroup$
– JulianGi
Dec 13 '18 at 13:46
1
1
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
$begingroup$
@JulianGi recall divisibility, remainder is $0leq r <n$. We excluded $0$ so $1leq r <n$ or we have $n-1$ possible values from $1$ to $n-1$. The sequence constructed $a_1, a_1+a_2,...,a_1+a_2+...+a_n$ contains $n$ elements, taking the remainder of $pmod{n}$ yields $n$ remainders, taking values from $1$ to $n-1$. There is no one to one mapping between $n$ remainders taking values from ${1,2,...n-1}$ and ${1,2,...n-1}$, so "at least 2 (remainders) will have the same value ...".
$endgroup$
– rtybase
Dec 13 '18 at 14:01
1
1
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
$begingroup$
@rtybase That makes a lot of sense. Thank you very much.
$endgroup$
– JulianGi
Dec 13 '18 at 14:07
add a comment |
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