Prove that $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number












0














Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.



I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?










share|cite|improve this question


















  • 2




    It's not when $n=1$. Have you copied the question correctly?
    – Lord Shark the Unknown
    Nov 22 at 21:27










  • yes now i check but even for me it is so strange
    – Marko Škorić
    Nov 22 at 21:31










  • It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
    – fleablood
    Nov 22 at 21:32












  • You know that $n^3/6+n^2/2+n/3$ is an integer?
    – Lord Shark the Unknown
    Nov 22 at 21:32










  • See math.stackexchange.com/questions/710361/…
    – lab bhattacharjee
    Nov 23 at 4:09
















0














Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.



I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?










share|cite|improve this question


















  • 2




    It's not when $n=1$. Have you copied the question correctly?
    – Lord Shark the Unknown
    Nov 22 at 21:27










  • yes now i check but even for me it is so strange
    – Marko Škorić
    Nov 22 at 21:31










  • It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
    – fleablood
    Nov 22 at 21:32












  • You know that $n^3/6+n^2/2+n/3$ is an integer?
    – Lord Shark the Unknown
    Nov 22 at 21:32










  • See math.stackexchange.com/questions/710361/…
    – lab bhattacharjee
    Nov 23 at 4:09














0












0








0







Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.



I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?










share|cite|improve this question













Prove that for every $n in mathbb N$, $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$ is natural number.



I try in this form $frac{2n+2n^2+n^3}{6}=frac{n(n^2+2n+2)}{6}$ so I must show that $2| n(n^2+2n+2)$ and $3|n(n^2+2n+2)$ then I try that $n=2k$ or $n=2k+1$, $kin mathbb N$ to prove that if $n=2k$ it is trivial if $n=2k+1$ then $(2k+1)((2k+2)^2+1)$ then it is odd number multiply odd number so I did not prove that 2 divide that number, the same is for 3 do you have some idea?







discrete-mathematics modular-arithmetic divisibility






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Nov 22 at 21:24









Marko Škorić

70310




70310








  • 2




    It's not when $n=1$. Have you copied the question correctly?
    – Lord Shark the Unknown
    Nov 22 at 21:27










  • yes now i check but even for me it is so strange
    – Marko Škorić
    Nov 22 at 21:31










  • It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
    – fleablood
    Nov 22 at 21:32












  • You know that $n^3/6+n^2/2+n/3$ is an integer?
    – Lord Shark the Unknown
    Nov 22 at 21:32










  • See math.stackexchange.com/questions/710361/…
    – lab bhattacharjee
    Nov 23 at 4:09














  • 2




    It's not when $n=1$. Have you copied the question correctly?
    – Lord Shark the Unknown
    Nov 22 at 21:27










  • yes now i check but even for me it is so strange
    – Marko Škorić
    Nov 22 at 21:31










  • It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
    – fleablood
    Nov 22 at 21:32












  • You know that $n^3/6+n^2/2+n/3$ is an integer?
    – Lord Shark the Unknown
    Nov 22 at 21:32










  • See math.stackexchange.com/questions/710361/…
    – lab bhattacharjee
    Nov 23 at 4:09








2




2




It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27




It's not when $n=1$. Have you copied the question correctly?
– Lord Shark the Unknown
Nov 22 at 21:27












yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31




yes now i check but even for me it is so strange
– Marko Škorić
Nov 22 at 21:31












It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32






It seems it won't be true if n is odd. If n is 3 for example it is $1+3+frac 92$
– fleablood
Nov 22 at 21:32














You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32




You know that $n^3/6+n^2/2+n/3$ is an integer?
– Lord Shark the Unknown
Nov 22 at 21:32












See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09




See math.stackexchange.com/questions/710361/…
– lab bhattacharjee
Nov 23 at 4:09










4 Answers
4






active

oldest

votes


















1














By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$





Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's



Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$



But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$






share|cite|improve this answer































    3














    It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$






    share|cite|improve this answer





















    • yep, I suspect the same
      – Masacroso
      Nov 22 at 21:35










    • Just requires some manipulation in numerator or denominators........
      – Mostafa Ayaz
      Nov 22 at 21:36



















    2














    $m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.






    share|cite|improve this answer





























      0














      Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
      $$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
      takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.






      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%2f3009678%2fprove-that-fracn3-fracn23-fracn36-is-natural-number%23new-answer', 'question_page');
        }
        );

        Post as a guest















        Required, but never shown

























        4 Answers
        4






        active

        oldest

        votes








        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        1














        By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$





        Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's



        Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$



        But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$






        share|cite|improve this answer




























          1














          By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$





          Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's



          Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$



          But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$






          share|cite|improve this answer


























            1












            1








            1






            By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$





            Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's



            Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$



            But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$






            share|cite|improve this answer














            By below $,6mid nf(n)!=! n(n^2!+3n+2) $ by $ 6mid f(1)!=6,,$ and $,3mid f(-1)!=!0$





            Theorem $ forall n!: 6mid nf(n)! iff! 6mid f(1), 3mid f(-1) $ for a polynomial $f(x)$ with integer coef's



            Proof $ $ It is true $iff nf(n),$ has roots $,nequiv 0,1pmod{! 2}$ and $,nequiv 0,pm1 pmod{!3}$



            But $,0,$ is always a root, and $,1,$ is a root $!iff!!! underbrace{6mid f(1)}_{large 2,3 mid f(1) }!$ and $,-1$ is a root $iff!3mid f(-1)$







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Nov 22 at 23:21

























            answered Nov 22 at 22:17









            Bill Dubuque

            208k29190626




            208k29190626























                3














                It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$






                share|cite|improve this answer





















                • yep, I suspect the same
                  – Masacroso
                  Nov 22 at 21:35










                • Just requires some manipulation in numerator or denominators........
                  – Mostafa Ayaz
                  Nov 22 at 21:36
















                3














                It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$






                share|cite|improve this answer





















                • yep, I suspect the same
                  – Masacroso
                  Nov 22 at 21:35










                • Just requires some manipulation in numerator or denominators........
                  – Mostafa Ayaz
                  Nov 22 at 21:36














                3












                3








                3






                It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$






                share|cite|improve this answer












                It is a typo because $$frac{n}{3}+frac{n^2}{2}+frac{n^3}{6}={n(n+1)(n+2)over 6}$$ is natural number for all $ninBbb N$ not $frac{n}{3}+frac{n^2}{3}+frac{n^3}{6}$







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Nov 22 at 21:33









                Mostafa Ayaz

                13.7k3836




                13.7k3836












                • yep, I suspect the same
                  – Masacroso
                  Nov 22 at 21:35










                • Just requires some manipulation in numerator or denominators........
                  – Mostafa Ayaz
                  Nov 22 at 21:36


















                • yep, I suspect the same
                  – Masacroso
                  Nov 22 at 21:35










                • Just requires some manipulation in numerator or denominators........
                  – Mostafa Ayaz
                  Nov 22 at 21:36
















                yep, I suspect the same
                – Masacroso
                Nov 22 at 21:35




                yep, I suspect the same
                – Masacroso
                Nov 22 at 21:35












                Just requires some manipulation in numerator or denominators........
                – Mostafa Ayaz
                Nov 22 at 21:36




                Just requires some manipulation in numerator or denominators........
                – Mostafa Ayaz
                Nov 22 at 21:36











                2














                $m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.






                share|cite|improve this answer


























                  2














                  $m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.






                  share|cite|improve this answer
























                    2












                    2








                    2






                    $m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.






                    share|cite|improve this answer












                    $m=n/3+n^2/3+n^3/6=n(n^2+2n+2)/6=n((n+1)^2+1)/6$. If $n$ is odd then so is $(n+1)^2+1.$ So, for $n$ odd $m$ is not an integer.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Nov 22 at 21:32









                    John_Wick

                    1,356111




                    1,356111























                        0














                        Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
                        $$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
                        takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.






                        share|cite|improve this answer


























                          0














                          Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
                          $$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
                          takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.






                          share|cite|improve this answer
























                            0












                            0








                            0






                            Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
                            $$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
                            takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.






                            share|cite|improve this answer












                            Insane overkill incoming: if a polynomial $p(x)$ with degree $d$ takes integer values at $xin{0,1,ldots,d}$, it takes integer values at any $xinmathbb{Z}$, as a consequence of the properties of the forward difference operator $delta:p(x)mapsto p(x+1)-p(x)$. Since
                            $$ frac{n}{3}+frac{n^2}{color{red}{2}}+frac{n^3}{6} $$
                            takes integer values at ${0,1,2,3}$, it takes integer values at any $ninmathbb{N}$, and these values are clearly non-negative.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Nov 22 at 22:32









                            Jack D'Aurizio

                            286k33279655




                            286k33279655






























                                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%2f3009678%2fprove-that-fracn3-fracn23-fracn36-is-natural-number%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

                                Plaza Victoria

                                Brian Clough

                                Cáceres