Are the elements of this set distinct? Can it be considered a set if they are not?












0












$begingroup$


I'm currently reading Book of Proof by Richard Hammack. In the chapter on sets, he gives this as an example:




X = { n2 : n ∈ Z }




If n can be any integer and (-n)2 = (n)2, are the elements of this set distinct? If they are not, is this a set?



To my understanding, sets cannot have repeated elements. However, on the wikipedia page for sets it defines a set as




a collection of distinct objects, considered as an object in its own right




but later says




In an extensional definition, a set member can be listed two or more times, for example, {11, 6, 6}.




So can sets have repeated elements?










share|cite|improve this question









$endgroup$

















    0












    $begingroup$


    I'm currently reading Book of Proof by Richard Hammack. In the chapter on sets, he gives this as an example:




    X = { n2 : n ∈ Z }




    If n can be any integer and (-n)2 = (n)2, are the elements of this set distinct? If they are not, is this a set?



    To my understanding, sets cannot have repeated elements. However, on the wikipedia page for sets it defines a set as




    a collection of distinct objects, considered as an object in its own right




    but later says




    In an extensional definition, a set member can be listed two or more times, for example, {11, 6, 6}.




    So can sets have repeated elements?










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      I'm currently reading Book of Proof by Richard Hammack. In the chapter on sets, he gives this as an example:




      X = { n2 : n ∈ Z }




      If n can be any integer and (-n)2 = (n)2, are the elements of this set distinct? If they are not, is this a set?



      To my understanding, sets cannot have repeated elements. However, on the wikipedia page for sets it defines a set as




      a collection of distinct objects, considered as an object in its own right




      but later says




      In an extensional definition, a set member can be listed two or more times, for example, {11, 6, 6}.




      So can sets have repeated elements?










      share|cite|improve this question









      $endgroup$




      I'm currently reading Book of Proof by Richard Hammack. In the chapter on sets, he gives this as an example:




      X = { n2 : n ∈ Z }




      If n can be any integer and (-n)2 = (n)2, are the elements of this set distinct? If they are not, is this a set?



      To my understanding, sets cannot have repeated elements. However, on the wikipedia page for sets it defines a set as




      a collection of distinct objects, considered as an object in its own right




      but later says




      In an extensional definition, a set member can be listed two or more times, for example, {11, 6, 6}.




      So can sets have repeated elements?







      elementary-set-theory






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Dec 4 '18 at 14:16









      Copeland CorleyCopeland Corley

      82




      82






















          3 Answers
          3






          active

          oldest

          votes


















          0












          $begingroup$

          Think of it like this: an element $x$ of a set $A$ can be mentioned in the notation of the set more than once.



          Here ${11,6,6}$ is a notation of the unique set that only has the elements $6$ and $11$ (and is completely determined by that fact) and in this notation element $6$ is mentioned twice. That is allowed.



          That's all.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:36










          • $begingroup$
            By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
            $endgroup$
            – drhab
            Dec 4 '18 at 14:58












          • $begingroup$
            One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 15:33










          • $begingroup$
            Yes, they do. The cardinality is $2$ as you will understand.
            $endgroup$
            – drhab
            Dec 4 '18 at 16:40



















          0












          $begingroup$

          Yes, it is a set. Note that ${1,1}$ and ${1}$ are the same set. So$$X={1,1,4,4,9,9,16,16,ldots}={1,4,9,16,ldots}$$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:39










          • $begingroup$
            I have no experience whatsoever with multisets.
            $endgroup$
            – José Carlos Santos
            Dec 4 '18 at 14:40



















          0












          $begingroup$

          The notation allows to repeat the elements, but as they are identical, they "count once" only. So, strictly,



          $${11,6,6}={11,6}.$$



          True replication requires a multiset.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:37










          • $begingroup$
            @CopelandCorley: of course. Multisets aren't so often met.
            $endgroup$
            – Yves Daoust
            Dec 4 '18 at 14:55











          Your Answer





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          3 Answers
          3






          active

          oldest

          votes








          3 Answers
          3






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          0












          $begingroup$

          Think of it like this: an element $x$ of a set $A$ can be mentioned in the notation of the set more than once.



          Here ${11,6,6}$ is a notation of the unique set that only has the elements $6$ and $11$ (and is completely determined by that fact) and in this notation element $6$ is mentioned twice. That is allowed.



          That's all.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:36










          • $begingroup$
            By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
            $endgroup$
            – drhab
            Dec 4 '18 at 14:58












          • $begingroup$
            One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 15:33










          • $begingroup$
            Yes, they do. The cardinality is $2$ as you will understand.
            $endgroup$
            – drhab
            Dec 4 '18 at 16:40
















          0












          $begingroup$

          Think of it like this: an element $x$ of a set $A$ can be mentioned in the notation of the set more than once.



          Here ${11,6,6}$ is a notation of the unique set that only has the elements $6$ and $11$ (and is completely determined by that fact) and in this notation element $6$ is mentioned twice. That is allowed.



          That's all.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:36










          • $begingroup$
            By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
            $endgroup$
            – drhab
            Dec 4 '18 at 14:58












          • $begingroup$
            One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 15:33










          • $begingroup$
            Yes, they do. The cardinality is $2$ as you will understand.
            $endgroup$
            – drhab
            Dec 4 '18 at 16:40














          0












          0








          0





          $begingroup$

          Think of it like this: an element $x$ of a set $A$ can be mentioned in the notation of the set more than once.



          Here ${11,6,6}$ is a notation of the unique set that only has the elements $6$ and $11$ (and is completely determined by that fact) and in this notation element $6$ is mentioned twice. That is allowed.



          That's all.






          share|cite|improve this answer









          $endgroup$



          Think of it like this: an element $x$ of a set $A$ can be mentioned in the notation of the set more than once.



          Here ${11,6,6}$ is a notation of the unique set that only has the elements $6$ and $11$ (and is completely determined by that fact) and in this notation element $6$ is mentioned twice. That is allowed.



          That's all.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 4 '18 at 14:27









          drhabdrhab

          102k545136




          102k545136












          • $begingroup$
            So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:36










          • $begingroup$
            By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
            $endgroup$
            – drhab
            Dec 4 '18 at 14:58












          • $begingroup$
            One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 15:33










          • $begingroup$
            Yes, they do. The cardinality is $2$ as you will understand.
            $endgroup$
            – drhab
            Dec 4 '18 at 16:40


















          • $begingroup$
            So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:36










          • $begingroup$
            By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
            $endgroup$
            – drhab
            Dec 4 '18 at 14:58












          • $begingroup$
            One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 15:33










          • $begingroup$
            Yes, they do. The cardinality is $2$ as you will understand.
            $endgroup$
            – drhab
            Dec 4 '18 at 16:40
















          $begingroup$
          So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:36




          $begingroup$
          So how would I distinguish a set that mentions an element multiple times and a multiset that contains multiple identical objects that are separate elementst?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:36












          $begingroup$
          By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
          $endgroup$
          – drhab
          Dec 4 '18 at 14:58






          $begingroup$
          By paying attention to the context that you are working in, I would say. Multisets and sets are distinct mathematical objects. Also see here where is mentioned e.g."...to distinguish between sets and multisets, a notation that incorporates square brackets is sometimes used: the multiset ${a,a,b}$ can be denoted as $[a, a, b]$...". I assure you that if you once work with multisets then you will certainly be aware of that too :-). In your question you only mention sets and not multisets.
          $endgroup$
          – drhab
          Dec 4 '18 at 14:58














          $begingroup$
          One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 15:33




          $begingroup$
          One final question. Since {11, 6} and {11, 6, 6} are the same set, do they have the same cardinality?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 15:33












          $begingroup$
          Yes, they do. The cardinality is $2$ as you will understand.
          $endgroup$
          – drhab
          Dec 4 '18 at 16:40




          $begingroup$
          Yes, they do. The cardinality is $2$ as you will understand.
          $endgroup$
          – drhab
          Dec 4 '18 at 16:40











          0












          $begingroup$

          Yes, it is a set. Note that ${1,1}$ and ${1}$ are the same set. So$$X={1,1,4,4,9,9,16,16,ldots}={1,4,9,16,ldots}$$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:39










          • $begingroup$
            I have no experience whatsoever with multisets.
            $endgroup$
            – José Carlos Santos
            Dec 4 '18 at 14:40
















          0












          $begingroup$

          Yes, it is a set. Note that ${1,1}$ and ${1}$ are the same set. So$$X={1,1,4,4,9,9,16,16,ldots}={1,4,9,16,ldots}$$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:39










          • $begingroup$
            I have no experience whatsoever with multisets.
            $endgroup$
            – José Carlos Santos
            Dec 4 '18 at 14:40














          0












          0








          0





          $begingroup$

          Yes, it is a set. Note that ${1,1}$ and ${1}$ are the same set. So$$X={1,1,4,4,9,9,16,16,ldots}={1,4,9,16,ldots}$$






          share|cite|improve this answer









          $endgroup$



          Yes, it is a set. Note that ${1,1}$ and ${1}$ are the same set. So$$X={1,1,4,4,9,9,16,16,ldots}={1,4,9,16,ldots}$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 4 '18 at 14:19









          José Carlos SantosJosé Carlos Santos

          164k22131234




          164k22131234












          • $begingroup$
            Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:39










          • $begingroup$
            I have no experience whatsoever with multisets.
            $endgroup$
            – José Carlos Santos
            Dec 4 '18 at 14:40


















          • $begingroup$
            Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:39










          • $begingroup$
            I have no experience whatsoever with multisets.
            $endgroup$
            – José Carlos Santos
            Dec 4 '18 at 14:40
















          $begingroup$
          Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:39




          $begingroup$
          Okay, so if I have the set {1,1} and the multiset {1,1}, they are not equal, right?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:39












          $begingroup$
          I have no experience whatsoever with multisets.
          $endgroup$
          – José Carlos Santos
          Dec 4 '18 at 14:40




          $begingroup$
          I have no experience whatsoever with multisets.
          $endgroup$
          – José Carlos Santos
          Dec 4 '18 at 14:40











          0












          $begingroup$

          The notation allows to repeat the elements, but as they are identical, they "count once" only. So, strictly,



          $${11,6,6}={11,6}.$$



          True replication requires a multiset.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:37










          • $begingroup$
            @CopelandCorley: of course. Multisets aren't so often met.
            $endgroup$
            – Yves Daoust
            Dec 4 '18 at 14:55
















          0












          $begingroup$

          The notation allows to repeat the elements, but as they are identical, they "count once" only. So, strictly,



          $${11,6,6}={11,6}.$$



          True replication requires a multiset.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:37










          • $begingroup$
            @CopelandCorley: of course. Multisets aren't so often met.
            $endgroup$
            – Yves Daoust
            Dec 4 '18 at 14:55














          0












          0








          0





          $begingroup$

          The notation allows to repeat the elements, but as they are identical, they "count once" only. So, strictly,



          $${11,6,6}={11,6}.$$



          True replication requires a multiset.






          share|cite|improve this answer









          $endgroup$



          The notation allows to repeat the elements, but as they are identical, they "count once" only. So, strictly,



          $${11,6,6}={11,6}.$$



          True replication requires a multiset.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 4 '18 at 14:28









          Yves DaoustYves Daoust

          129k675227




          129k675227












          • $begingroup$
            How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:37










          • $begingroup$
            @CopelandCorley: of course. Multisets aren't so often met.
            $endgroup$
            – Yves Daoust
            Dec 4 '18 at 14:55


















          • $begingroup$
            How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
            $endgroup$
            – Copeland Corley
            Dec 4 '18 at 14:37










          • $begingroup$
            @CopelandCorley: of course. Multisets aren't so often met.
            $endgroup$
            – Yves Daoust
            Dec 4 '18 at 14:55
















          $begingroup$
          How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:37




          $begingroup$
          How would I know that I am dealing with a set instead of a multiset? Is it supposed to be inferred from the context?
          $endgroup$
          – Copeland Corley
          Dec 4 '18 at 14:37












          $begingroup$
          @CopelandCorley: of course. Multisets aren't so often met.
          $endgroup$
          – Yves Daoust
          Dec 4 '18 at 14:55




          $begingroup$
          @CopelandCorley: of course. Multisets aren't so often met.
          $endgroup$
          – Yves Daoust
          Dec 4 '18 at 14:55


















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