Do these $n$ cyclic conditions imply every two numbers are pairwise coprime?
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The question probably sounds vague in the title, so here is the question. Suppose that for a set of $n$ (not necessarily distinct) integers $S={x_1,x_2,x_3,ldots,x_n}$, it is true that all integers in $Ssetminus{x_k}$ are coprime for each $kin{1,2,ldots,n}$ (in the sense that no integer $d>1$ divides every element in $S$). Is it true that $operatorname{gcd}(x_i,x_j)=1$ for all $ineq j$ ?
Forgive me but I do not know how to proceed. I think this claim is true, since the $n$ sets of criteria seem to be sufficient to imply this. But I have no idea how to actually prove it. I thought that since no integer $d>1$ divides all of ${x_1,ldots,x_{n-1}}$, then surely there must be two numbers in this set which are coprime. This would immediately imply the conclusion, but I have no clue how to prove that claim either. Any help is appreciated, thanks!
number-theory elementary-number-theory
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The question probably sounds vague in the title, so here is the question. Suppose that for a set of $n$ (not necessarily distinct) integers $S={x_1,x_2,x_3,ldots,x_n}$, it is true that all integers in $Ssetminus{x_k}$ are coprime for each $kin{1,2,ldots,n}$ (in the sense that no integer $d>1$ divides every element in $S$). Is it true that $operatorname{gcd}(x_i,x_j)=1$ for all $ineq j$ ?
Forgive me but I do not know how to proceed. I think this claim is true, since the $n$ sets of criteria seem to be sufficient to imply this. But I have no idea how to actually prove it. I thought that since no integer $d>1$ divides all of ${x_1,ldots,x_{n-1}}$, then surely there must be two numbers in this set which are coprime. This would immediately imply the conclusion, but I have no clue how to prove that claim either. Any help is appreciated, thanks!
number-theory elementary-number-theory
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
The question probably sounds vague in the title, so here is the question. Suppose that for a set of $n$ (not necessarily distinct) integers $S={x_1,x_2,x_3,ldots,x_n}$, it is true that all integers in $Ssetminus{x_k}$ are coprime for each $kin{1,2,ldots,n}$ (in the sense that no integer $d>1$ divides every element in $S$). Is it true that $operatorname{gcd}(x_i,x_j)=1$ for all $ineq j$ ?
Forgive me but I do not know how to proceed. I think this claim is true, since the $n$ sets of criteria seem to be sufficient to imply this. But I have no idea how to actually prove it. I thought that since no integer $d>1$ divides all of ${x_1,ldots,x_{n-1}}$, then surely there must be two numbers in this set which are coprime. This would immediately imply the conclusion, but I have no clue how to prove that claim either. Any help is appreciated, thanks!
number-theory elementary-number-theory
The question probably sounds vague in the title, so here is the question. Suppose that for a set of $n$ (not necessarily distinct) integers $S={x_1,x_2,x_3,ldots,x_n}$, it is true that all integers in $Ssetminus{x_k}$ are coprime for each $kin{1,2,ldots,n}$ (in the sense that no integer $d>1$ divides every element in $S$). Is it true that $operatorname{gcd}(x_i,x_j)=1$ for all $ineq j$ ?
Forgive me but I do not know how to proceed. I think this claim is true, since the $n$ sets of criteria seem to be sufficient to imply this. But I have no idea how to actually prove it. I thought that since no integer $d>1$ divides all of ${x_1,ldots,x_{n-1}}$, then surely there must be two numbers in this set which are coprime. This would immediately imply the conclusion, but I have no clue how to prove that claim either. Any help is appreciated, thanks!
number-theory elementary-number-theory
number-theory elementary-number-theory
edited Nov 16 at 12:40
asked Nov 16 at 12:31
YiFan
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1,5041311
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2 Answers
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It is false for $n=2$ as in the other answer - take $S={2,4}$.
It is true for $n=3$ because your assumptions just mean all pairs are coprime.
It is false for $ngeq 4$. Take $S={p_1p_2,p_2p_3,ldots,p_np_1}$ where $p_i$ are distinct primes.
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This is wrong if $n=2$ - for instance, consider $S = lbrace 2, 4 rbrace$.
Otherwise, given $i neq j$, we can find $k notin lbrace i, j rbrace$ and since all integers in $S setminus lbrace x_k rbrace$ are coprime, $x_i$ and $x_j$ are.
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
It is false for $n=2$ as in the other answer - take $S={2,4}$.
It is true for $n=3$ because your assumptions just mean all pairs are coprime.
It is false for $ngeq 4$. Take $S={p_1p_2,p_2p_3,ldots,p_np_1}$ where $p_i$ are distinct primes.
add a comment |
up vote
2
down vote
accepted
It is false for $n=2$ as in the other answer - take $S={2,4}$.
It is true for $n=3$ because your assumptions just mean all pairs are coprime.
It is false for $ngeq 4$. Take $S={p_1p_2,p_2p_3,ldots,p_np_1}$ where $p_i$ are distinct primes.
add a comment |
up vote
2
down vote
accepted
up vote
2
down vote
accepted
It is false for $n=2$ as in the other answer - take $S={2,4}$.
It is true for $n=3$ because your assumptions just mean all pairs are coprime.
It is false for $ngeq 4$. Take $S={p_1p_2,p_2p_3,ldots,p_np_1}$ where $p_i$ are distinct primes.
It is false for $n=2$ as in the other answer - take $S={2,4}$.
It is true for $n=3$ because your assumptions just mean all pairs are coprime.
It is false for $ngeq 4$. Take $S={p_1p_2,p_2p_3,ldots,p_np_1}$ where $p_i$ are distinct primes.
answered Nov 16 at 12:43
Michal Adamaszek
2,02648
2,02648
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This is wrong if $n=2$ - for instance, consider $S = lbrace 2, 4 rbrace$.
Otherwise, given $i neq j$, we can find $k notin lbrace i, j rbrace$ and since all integers in $S setminus lbrace x_k rbrace$ are coprime, $x_i$ and $x_j$ are.
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
add a comment |
up vote
0
down vote
This is wrong if $n=2$ - for instance, consider $S = lbrace 2, 4 rbrace$.
Otherwise, given $i neq j$, we can find $k notin lbrace i, j rbrace$ and since all integers in $S setminus lbrace x_k rbrace$ are coprime, $x_i$ and $x_j$ are.
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
add a comment |
up vote
0
down vote
up vote
0
down vote
This is wrong if $n=2$ - for instance, consider $S = lbrace 2, 4 rbrace$.
Otherwise, given $i neq j$, we can find $k notin lbrace i, j rbrace$ and since all integers in $S setminus lbrace x_k rbrace$ are coprime, $x_i$ and $x_j$ are.
This is wrong if $n=2$ - for instance, consider $S = lbrace 2, 4 rbrace$.
Otherwise, given $i neq j$, we can find $k notin lbrace i, j rbrace$ and since all integers in $S setminus lbrace x_k rbrace$ are coprime, $x_i$ and $x_j$ are.
answered Nov 16 at 12:36
Stockfish
40226
40226
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
add a comment |
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Sorry if my description was unclear. By 'all integers in $Ssetminus{x_k}$ are coprime, I do not mean pairwise coprime, like what we're trying to prove. I mean that there is no integer $d>1$ so that $dmid s$ for all $sin S$.
– YiFan
Nov 16 at 12:39
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
Ah, I see. I guess we still need $n>2$ though.
– Stockfish
Nov 16 at 12:41
add a comment |
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