Union of a finite set and a countably infinite set is countably infinite












2














Ok, here is the problem statement:




Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.




This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!










share|cite|improve this question
























  • What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
    – Ian Coley
    Apr 7 '13 at 23:30
















2














Ok, here is the problem statement:




Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.




This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!










share|cite|improve this question
























  • What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
    – Ian Coley
    Apr 7 '13 at 23:30














2












2








2







Ok, here is the problem statement:




Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.




This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!










share|cite|improve this question















Ok, here is the problem statement:




Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.




This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!







elementary-set-theory






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Apr 7 '13 at 23:30









Zev Chonoles

109k16226422




109k16226422










asked Apr 7 '13 at 23:26









user69839user69839

6327




6327












  • What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
    – Ian Coley
    Apr 7 '13 at 23:30


















  • What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
    – Ian Coley
    Apr 7 '13 at 23:30
















What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30




What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30










2 Answers
2






active

oldest

votes


















4














Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.



Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...






share|cite|improve this answer





















  • Thank you! I think I got it
    – user69839
    Apr 7 '13 at 23:34



















1














It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.






share|cite|improve this answer





















    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "69"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f354351%2funion-of-a-finite-set-and-a-countably-infinite-set-is-countably-infinite%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    4














    Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.



    Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
    $$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
    That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...






    share|cite|improve this answer





















    • Thank you! I think I got it
      – user69839
      Apr 7 '13 at 23:34
















    4














    Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.



    Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
    $$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
    That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...






    share|cite|improve this answer





















    • Thank you! I think I got it
      – user69839
      Apr 7 '13 at 23:34














    4












    4








    4






    Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.



    Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
    $$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
    That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...






    share|cite|improve this answer












    Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.



    Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
    $$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
    That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Apr 7 '13 at 23:31









    BerciBerci

    59.7k23672




    59.7k23672












    • Thank you! I think I got it
      – user69839
      Apr 7 '13 at 23:34


















    • Thank you! I think I got it
      – user69839
      Apr 7 '13 at 23:34
















    Thank you! I think I got it
    – user69839
    Apr 7 '13 at 23:34




    Thank you! I think I got it
    – user69839
    Apr 7 '13 at 23:34











    1














    It is simple to prove that the countable union of countable sets is countable:
    Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.






    share|cite|improve this answer


























      1














      It is simple to prove that the countable union of countable sets is countable:
      Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.






      share|cite|improve this answer
























        1












        1








        1






        It is simple to prove that the countable union of countable sets is countable:
        Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.






        share|cite|improve this answer












        It is simple to prove that the countable union of countable sets is countable:
        Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Apr 7 '13 at 23:44









        vonbrandvonbrand

        19.8k63058




        19.8k63058






























            draft saved

            draft discarded




















































            Thanks for contributing an answer to Mathematics Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.





            Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


            Please pay close attention to the following guidance:


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f354351%2funion-of-a-finite-set-and-a-countably-infinite-set-is-countably-infinite%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            Ellipse (mathématiques)

            Quarter-circle Tiles

            Mont Emei