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!










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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!










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      up vote
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      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!










      share|cite|improve this question















      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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      edited Nov 16 at 12:40

























      asked Nov 16 at 12:31









      YiFan

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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.






            share|cite|improve this answer





















            • 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
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            active

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            2 Answers
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            active

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            active

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            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.






            share|cite|improve this answer

























              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.






              share|cite|improve this answer























                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.






                share|cite|improve this answer












                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.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Nov 16 at 12:43









                Michal Adamaszek

                2,02648




                2,02648






















                    up vote
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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.






                    share|cite|improve this answer





















                    • 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















                    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.






                    share|cite|improve this answer





















                    • 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













                    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.






                    share|cite|improve this answer












                    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.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    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


















                    • 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


















                     

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