Calculate the probability with a finite arithmetic progression












1












$begingroup$


We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.



My proposition :



$ Omega={n!over (n-3)!}$



$ mathbf{A}= {n choose 3} cdot 2$



But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.



Any suggestions how can I count it? Thanks in advice, Kuba!










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Is the progression finite?
    $endgroup$
    – EuxhenH
    Jan 7 at 18:05










  • $begingroup$
    I totally forgot about it :/. Yes, the progression is finite.
    $endgroup$
    – Kukoz
    Jan 7 at 18:15
















1












$begingroup$


We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.



My proposition :



$ Omega={n!over (n-3)!}$



$ mathbf{A}= {n choose 3} cdot 2$



But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.



Any suggestions how can I count it? Thanks in advice, Kuba!










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Is the progression finite?
    $endgroup$
    – EuxhenH
    Jan 7 at 18:05










  • $begingroup$
    I totally forgot about it :/. Yes, the progression is finite.
    $endgroup$
    – Kukoz
    Jan 7 at 18:15














1












1








1





$begingroup$


We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.



My proposition :



$ Omega={n!over (n-3)!}$



$ mathbf{A}= {n choose 3} cdot 2$



But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.



Any suggestions how can I count it? Thanks in advice, Kuba!










share|cite|improve this question











$endgroup$




We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.



My proposition :



$ Omega={n!over (n-3)!}$



$ mathbf{A}= {n choose 3} cdot 2$



But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.



Any suggestions how can I count it? Thanks in advice, Kuba!







probability problem-solving arithmetic-progressions






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 7 at 19:14







Kukoz

















asked Jan 7 at 17:28









KukozKukoz

409




409








  • 2




    $begingroup$
    Is the progression finite?
    $endgroup$
    – EuxhenH
    Jan 7 at 18:05










  • $begingroup$
    I totally forgot about it :/. Yes, the progression is finite.
    $endgroup$
    – Kukoz
    Jan 7 at 18:15














  • 2




    $begingroup$
    Is the progression finite?
    $endgroup$
    – EuxhenH
    Jan 7 at 18:05










  • $begingroup$
    I totally forgot about it :/. Yes, the progression is finite.
    $endgroup$
    – Kukoz
    Jan 7 at 18:15








2




2




$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05




$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05












$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15




$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15










2 Answers
2






active

oldest

votes


















1












$begingroup$

$Omega = n(n-1)(n-2)$



We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$

Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.

My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
    $endgroup$
    – Kukoz
    Jan 10 at 14:41








  • 1




    $begingroup$
    No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
    $endgroup$
    – The Cat
    Jan 10 at 16:28






  • 1




    $begingroup$
    $lceil 0.5rceil $=1
    $endgroup$
    – The Cat
    Jan 10 at 16:29








  • 1




    $begingroup$
    I think you just mistook those two together.
    $endgroup$
    – The Cat
    Jan 10 at 16:30






  • 1




    $begingroup$
    Yeah, now everything is good! Big thanks for you, and shoutout!
    $endgroup$
    – Kukoz
    Jan 10 at 16:33



















0












$begingroup$

Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$



and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.



So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$



If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$






share|cite|improve this answer









$endgroup$













    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%2f3065253%2fcalculate-the-probability-with-a-finite-arithmetic-progression%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









    1












    $begingroup$

    $Omega = n(n-1)(n-2)$



    We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$

    Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.

    My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
    $P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$






    share|cite|improve this answer











    $endgroup$









    • 1




      $begingroup$
      Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
      $endgroup$
      – Kukoz
      Jan 10 at 14:41








    • 1




      $begingroup$
      No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
      $endgroup$
      – The Cat
      Jan 10 at 16:28






    • 1




      $begingroup$
      $lceil 0.5rceil $=1
      $endgroup$
      – The Cat
      Jan 10 at 16:29








    • 1




      $begingroup$
      I think you just mistook those two together.
      $endgroup$
      – The Cat
      Jan 10 at 16:30






    • 1




      $begingroup$
      Yeah, now everything is good! Big thanks for you, and shoutout!
      $endgroup$
      – Kukoz
      Jan 10 at 16:33
















    1












    $begingroup$

    $Omega = n(n-1)(n-2)$



    We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$

    Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.

    My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
    $P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$






    share|cite|improve this answer











    $endgroup$









    • 1




      $begingroup$
      Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
      $endgroup$
      – Kukoz
      Jan 10 at 14:41








    • 1




      $begingroup$
      No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
      $endgroup$
      – The Cat
      Jan 10 at 16:28






    • 1




      $begingroup$
      $lceil 0.5rceil $=1
      $endgroup$
      – The Cat
      Jan 10 at 16:29








    • 1




      $begingroup$
      I think you just mistook those two together.
      $endgroup$
      – The Cat
      Jan 10 at 16:30






    • 1




      $begingroup$
      Yeah, now everything is good! Big thanks for you, and shoutout!
      $endgroup$
      – Kukoz
      Jan 10 at 16:33














    1












    1








    1





    $begingroup$

    $Omega = n(n-1)(n-2)$



    We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$

    Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.

    My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
    $P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$






    share|cite|improve this answer











    $endgroup$



    $Omega = n(n-1)(n-2)$



    We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$

    Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.

    My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
    $P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Jan 10 at 18:33

























    answered Jan 7 at 19:35









    The CatThe Cat

    25112




    25112








    • 1




      $begingroup$
      Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
      $endgroup$
      – Kukoz
      Jan 10 at 14:41








    • 1




      $begingroup$
      No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
      $endgroup$
      – The Cat
      Jan 10 at 16:28






    • 1




      $begingroup$
      $lceil 0.5rceil $=1
      $endgroup$
      – The Cat
      Jan 10 at 16:29








    • 1




      $begingroup$
      I think you just mistook those two together.
      $endgroup$
      – The Cat
      Jan 10 at 16:30






    • 1




      $begingroup$
      Yeah, now everything is good! Big thanks for you, and shoutout!
      $endgroup$
      – Kukoz
      Jan 10 at 16:33














    • 1




      $begingroup$
      Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
      $endgroup$
      – Kukoz
      Jan 10 at 14:41








    • 1




      $begingroup$
      No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
      $endgroup$
      – The Cat
      Jan 10 at 16:28






    • 1




      $begingroup$
      $lceil 0.5rceil $=1
      $endgroup$
      – The Cat
      Jan 10 at 16:29








    • 1




      $begingroup$
      I think you just mistook those two together.
      $endgroup$
      – The Cat
      Jan 10 at 16:30






    • 1




      $begingroup$
      Yeah, now everything is good! Big thanks for you, and shoutout!
      $endgroup$
      – Kukoz
      Jan 10 at 16:33








    1




    1




    $begingroup$
    Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
    $endgroup$
    – Kukoz
    Jan 10 at 14:41






    $begingroup$
    Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
    $endgroup$
    – Kukoz
    Jan 10 at 14:41






    1




    1




    $begingroup$
    No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
    $endgroup$
    – The Cat
    Jan 10 at 16:28




    $begingroup$
    No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
    $endgroup$
    – The Cat
    Jan 10 at 16:28




    1




    1




    $begingroup$
    $lceil 0.5rceil $=1
    $endgroup$
    – The Cat
    Jan 10 at 16:29






    $begingroup$
    $lceil 0.5rceil $=1
    $endgroup$
    – The Cat
    Jan 10 at 16:29






    1




    1




    $begingroup$
    I think you just mistook those two together.
    $endgroup$
    – The Cat
    Jan 10 at 16:30




    $begingroup$
    I think you just mistook those two together.
    $endgroup$
    – The Cat
    Jan 10 at 16:30




    1




    1




    $begingroup$
    Yeah, now everything is good! Big thanks for you, and shoutout!
    $endgroup$
    – Kukoz
    Jan 10 at 16:33




    $begingroup$
    Yeah, now everything is good! Big thanks for you, and shoutout!
    $endgroup$
    – Kukoz
    Jan 10 at 16:33











    0












    $begingroup$

    Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$



    and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.



    So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
    good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$



    If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
    good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$






    share|cite|improve this answer









    $endgroup$


















      0












      $begingroup$

      Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$



      and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.



      So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
      good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$



      If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
      good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$






      share|cite|improve this answer









      $endgroup$
















        0












        0








        0





        $begingroup$

        Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$



        and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.



        So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
        good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$



        If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
        good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$






        share|cite|improve this answer









        $endgroup$



        Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$



        and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.



        So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
        good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$



        If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
        good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 10 at 17:36









        greedoidgreedoid

        42.9k1153105




        42.9k1153105






























            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.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3065253%2fcalculate-the-probability-with-a-finite-arithmetic-progression%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

            Human spaceflight

            Can not write log (Is /dev/pts mounted?) - openpty in Ubuntu-on-Windows?

            張江高科駅