Factorial inverse equation
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
add a comment |
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
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
add a comment |
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
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
factorial poisson-distribution
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
add a comment |
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
add a comment |
3 Answers
3
active
oldest
votes
Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:
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 $K$.
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
add a comment |
$x!$ 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.
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
add a comment |
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,
$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$
add a comment |
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
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:
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 $K$.
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
add a comment |
Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:
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 $K$.
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
add a comment |
Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:
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 $K$.
Before trying to solve this equation, I think it is better to get a sense of the possible solutions. Maybe this graph could help:
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 $K$.
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
add a comment |
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
add a comment |
$x!$ 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.
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
add a comment |
$x!$ 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.
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
add a comment |
$x!$ 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.
$x!$ 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.
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
add a comment |
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
add a comment |
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,
$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$
add a comment |
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,
$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$
add a comment |
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,
$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$
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,
$$ x ln x - (1 + ln A)x+frac{1}{2} ln{2pi x} + (A + ln K) = 0.$$
answered Dec 19 '16 at 17:05
Nitin
2,4291024
2,4291024
add a comment |
add a comment |
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.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
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
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