Factorial inverse equation












0














I need to find a solution to a problem and I need to solve an equation. The following equation is the result of some manipulation.



I'm a little bit rusty and I can't see how to find a simple solution ( to be honest I can't find a solution at all ):



$e^{-A} cdot A^x = x! cdot K$



What I need is $x$. $A$ and $K$ are real, positive constant numbers an I'm expecting the solution would be real and positive. Does anyone has suggestions ?










share|cite|improve this question


















  • 1




    So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
    – Simply Beautiful Art
    Dec 19 '16 at 16:08












  • Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
    – Simply Beautiful Art
    Dec 19 '16 at 16:12










  • Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
    – haster8558
    Dec 19 '16 at 16:14






  • 1




    It makes it almost certain that there is no solution.
    – Robert Israel
    Dec 19 '16 at 16:14
















0














I need to find a solution to a problem and I need to solve an equation. The following equation is the result of some manipulation.



I'm a little bit rusty and I can't see how to find a simple solution ( to be honest I can't find a solution at all ):



$e^{-A} cdot A^x = x! cdot K$



What I need is $x$. $A$ and $K$ are real, positive constant numbers an I'm expecting the solution would be real and positive. Does anyone has suggestions ?










share|cite|improve this question


















  • 1




    So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
    – Simply Beautiful Art
    Dec 19 '16 at 16:08












  • Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
    – Simply Beautiful Art
    Dec 19 '16 at 16:12










  • Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
    – haster8558
    Dec 19 '16 at 16:14






  • 1




    It makes it almost certain that there is no solution.
    – Robert Israel
    Dec 19 '16 at 16:14














0












0








0







I need to find a solution to a problem and I need to solve an equation. The following equation is the result of some manipulation.



I'm a little bit rusty and I can't see how to find a simple solution ( to be honest I can't find a solution at all ):



$e^{-A} cdot A^x = x! cdot K$



What I need is $x$. $A$ and $K$ are real, positive constant numbers an I'm expecting the solution would be real and positive. Does anyone has suggestions ?










share|cite|improve this question













I need to find a solution to a problem and I need to solve an equation. The following equation is the result of some manipulation.



I'm a little bit rusty and I can't see how to find a simple solution ( to be honest I can't find a solution at all ):



$e^{-A} cdot A^x = x! cdot K$



What I need is $x$. $A$ and $K$ are real, positive constant numbers an I'm expecting the solution would be real and positive. Does anyone has suggestions ?







factorial poisson-distribution






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 19 '16 at 16:06









haster8558

43




43








  • 1




    So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
    – Simply Beautiful Art
    Dec 19 '16 at 16:08












  • Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
    – Simply Beautiful Art
    Dec 19 '16 at 16:12










  • Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
    – haster8558
    Dec 19 '16 at 16:14






  • 1




    It makes it almost certain that there is no solution.
    – Robert Israel
    Dec 19 '16 at 16:14














  • 1




    So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
    – Simply Beautiful Art
    Dec 19 '16 at 16:08












  • Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
    – Simply Beautiful Art
    Dec 19 '16 at 16:12










  • Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
    – haster8558
    Dec 19 '16 at 16:14






  • 1




    It makes it almost certain that there is no solution.
    – Robert Israel
    Dec 19 '16 at 16:14








1




1




So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
– Simply Beautiful Art
Dec 19 '16 at 16:08






So... are we using the Gamma function? Since it is very unlikely that $x$ is an integer...
– Simply Beautiful Art
Dec 19 '16 at 16:08














Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
– Simply Beautiful Art
Dec 19 '16 at 16:12




Note that if we restrict $x>0$, it is plenty likely that there may be no such solutions.
– Simply Beautiful Art
Dec 19 '16 at 16:12












Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
– haster8558
Dec 19 '16 at 16:14




Well, to be honest i don't know what the Gamma function is, but to be precise $x$ is a natural ( integer $>= 0$ ), because it rapresent the number of defect in an wafer of silicon. So it's reasonable to think to it as an integer bigger than zero. Does it help ?
– haster8558
Dec 19 '16 at 16:14




1




1




It makes it almost certain that there is no solution.
– Robert Israel
Dec 19 '16 at 16:14




It makes it almost certain that there is no solution.
– Robert Israel
Dec 19 '16 at 16:14










3 Answers
3






active

oldest

votes


















1














Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:



graph



As you see, the solution is not unique for some values of $A$ and $K$, while it doesn't have any solution for some positive values of $A$ and $K$.






share|cite|improve this answer





















  • Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
    – haster8558
    Dec 19 '16 at 17:44










  • @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
    – polfosol
    Dec 19 '16 at 18:14










  • Ah right. I was in rush. It's definitely better. Thanks!!!
    – haster8558
    Dec 19 '16 at 18:38



















0














$x!$ is not defined for arbitrary real positive $x$, just for nonnegative integers. You could replace it by $Gamma(x+1)$. But still your equation can't be solved in "closed form". Numerical methods might be tried.






share|cite|improve this answer





















  • Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
    – haster8558
    Dec 19 '16 at 16:19










  • Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
    – Robert Israel
    Dec 19 '16 at 20:36



















0














Here's a possible solution - not sure if it will be of particularly good numerical stability. Take logs:



$$ ln{(e^{-A} A^x)} = ln{(x! K)} implies - A + x ln{A} = ln{x!} + ln{K}.$$



You might be able to solve this numerically if you use a an accurate enough approximation for $ln x!,$ using the Stirling series:



$$ ln x! approx x ln x - x + frac{1}{2} ln{2pi x} + frac{1}{12 x} - frac{1}{360 x^3} + dots$$



so a solution will be close to the solution of, say,



$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$






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%2f2064789%2ffactorial-inverse-equation%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    1














    Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:



    graph



    As you see, the solution is not unique for some values of $A$ and $K$, while it doesn't have any solution for some positive values of $A$ and $K$.






    share|cite|improve this answer





















    • Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
      – haster8558
      Dec 19 '16 at 17:44










    • @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
      – polfosol
      Dec 19 '16 at 18:14










    • Ah right. I was in rush. It's definitely better. Thanks!!!
      – haster8558
      Dec 19 '16 at 18:38
















    1














    Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:



    graph



    As you see, the solution is not unique for some values of $A$ and $K$, while it doesn't have any solution for some positive values of $A$ and $K$.






    share|cite|improve this answer





















    • Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
      – haster8558
      Dec 19 '16 at 17:44










    • @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
      – polfosol
      Dec 19 '16 at 18:14










    • Ah right. I was in rush. It's definitely better. Thanks!!!
      – haster8558
      Dec 19 '16 at 18:38














    1












    1








    1






    Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:



    graph



    As you see, the solution is not unique for some values of $A$ and $K$, while it doesn't have any solution for some positive values of $A$ and $K$.






    share|cite|improve this answer












    Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:



    graph



    As you see, the solution is not unique for some values of $A$ and $K$, while it doesn't have any solution for some positive values of $A$ and $K$.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Dec 19 '16 at 17:14









    polfosol

    5,54931845




    5,54931845












    • Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
      – haster8558
      Dec 19 '16 at 17:44










    • @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
      – polfosol
      Dec 19 '16 at 18:14










    • Ah right. I was in rush. It's definitely better. Thanks!!!
      – haster8558
      Dec 19 '16 at 18:38


















    • Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
      – haster8558
      Dec 19 '16 at 17:44










    • @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
      – polfosol
      Dec 19 '16 at 18:14










    • Ah right. I was in rush. It's definitely better. Thanks!!!
      – haster8558
      Dec 19 '16 at 18:38
















    Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
    – haster8558
    Dec 19 '16 at 17:44




    Looking this I got an idea. I can plot the function $e^{-A} cdot A^y = x! cdot K$ and then plot the equation $x = y$. The solution is in the cross. The only problem is to find the values a reasonable range for $x$ and $y$ .
    – haster8558
    Dec 19 '16 at 17:44












    @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
    – polfosol
    Dec 19 '16 at 18:14




    @haster8558 Why not just plot the function $e^{-A} cdot A^x - x! cdot K$ and then find its roots?
    – polfosol
    Dec 19 '16 at 18:14












    Ah right. I was in rush. It's definitely better. Thanks!!!
    – haster8558
    Dec 19 '16 at 18:38




    Ah right. I was in rush. It's definitely better. Thanks!!!
    – haster8558
    Dec 19 '16 at 18:38











    0














    $x!$ is not defined for arbitrary real positive $x$, just for nonnegative integers. You could replace it by $Gamma(x+1)$. But still your equation can't be solved in "closed form". Numerical methods might be tried.






    share|cite|improve this answer





















    • Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
      – haster8558
      Dec 19 '16 at 16:19










    • Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
      – Robert Israel
      Dec 19 '16 at 20:36
















    0














    $x!$ is not defined for arbitrary real positive $x$, just for nonnegative integers. You could replace it by $Gamma(x+1)$. But still your equation can't be solved in "closed form". Numerical methods might be tried.






    share|cite|improve this answer





















    • Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
      – haster8558
      Dec 19 '16 at 16:19










    • Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
      – Robert Israel
      Dec 19 '16 at 20:36














    0












    0








    0






    $x!$ is not defined for arbitrary real positive $x$, just for nonnegative integers. You could replace it by $Gamma(x+1)$. But still your equation can't be solved in "closed form". Numerical methods might be tried.






    share|cite|improve this answer












    $x!$ is not defined for arbitrary real positive $x$, just for nonnegative integers. You could replace it by $Gamma(x+1)$. But still your equation can't be solved in "closed form". Numerical methods might be tried.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Dec 19 '16 at 16:13









    Robert Israel

    318k23207458




    318k23207458












    • Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
      – haster8558
      Dec 19 '16 at 16:19










    • Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
      – Robert Israel
      Dec 19 '16 at 20:36


















    • Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
      – haster8558
      Dec 19 '16 at 16:19










    • Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
      – Robert Israel
      Dec 19 '16 at 20:36
















    Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
    – haster8558
    Dec 19 '16 at 16:19




    Ok, so it isn't possible to get a solution in closed form but it has some solutions. Any idea how to find a numerical solution ? Any algorithm simple to implement ?
    – haster8558
    Dec 19 '16 at 16:19












    Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
    – Robert Israel
    Dec 19 '16 at 20:36




    Newton's method should work. It might be better to work with the logarithms of both sides of the equation, to avoid numerical overflow.
    – Robert Israel
    Dec 19 '16 at 20:36











    0














    Here's a possible solution - not sure if it will be of particularly good numerical stability. Take logs:



    $$ ln{(e^{-A} A^x)} = ln{(x! K)} implies - A + x ln{A} = ln{x!} + ln{K}.$$



    You might be able to solve this numerically if you use a an accurate enough approximation for $ln x!,$ using the Stirling series:



    $$ ln x! approx x ln x - x + frac{1}{2} ln{2pi x} + frac{1}{12 x} - frac{1}{360 x^3} + dots$$



    so a solution will be close to the solution of, say,



    $$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$






    share|cite|improve this answer


























      0














      Here's a possible solution - not sure if it will be of particularly good numerical stability. Take logs:



      $$ ln{(e^{-A} A^x)} = ln{(x! K)} implies - A + x ln{A} = ln{x!} + ln{K}.$$



      You might be able to solve this numerically if you use a an accurate enough approximation for $ln x!,$ using the Stirling series:



      $$ ln x! approx x ln x - x + frac{1}{2} ln{2pi x} + frac{1}{12 x} - frac{1}{360 x^3} + dots$$



      so a solution will be close to the solution of, say,



      $$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$






      share|cite|improve this answer
























        0












        0








        0






        Here's a possible solution - not sure if it will be of particularly good numerical stability. Take logs:



        $$ ln{(e^{-A} A^x)} = ln{(x! K)} implies - A + x ln{A} = ln{x!} + ln{K}.$$



        You might be able to solve this numerically if you use a an accurate enough approximation for $ln x!,$ using the Stirling series:



        $$ ln x! approx x ln x - x + frac{1}{2} ln{2pi x} + frac{1}{12 x} - frac{1}{360 x^3} + dots$$



        so a solution will be close to the solution of, say,



        $$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$






        share|cite|improve this answer












        Here's a possible solution - not sure if it will be of particularly good numerical stability. Take logs:



        $$ ln{(e^{-A} A^x)} = ln{(x! K)} implies - A + x ln{A} = ln{x!} + ln{K}.$$



        You might be able to solve this numerically if you use a an accurate enough approximation for $ln x!,$ using the Stirling series:



        $$ ln x! approx x ln x - x + frac{1}{2} ln{2pi x} + frac{1}{12 x} - frac{1}{360 x^3} + dots$$



        so a solution will be close to the solution of, say,



        $$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 19 '16 at 17:05









        Nitin

        2,4291024




        2,4291024






























            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%2f2064789%2ffactorial-inverse-equation%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

            Wiesbaden

            Marschland

            Dieringhausen