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I am reading the article Deriving probability distributions using the Principle of Maximum Entropy and trying to derive some of the equations in it automatically using Mathematica.

I want to solve the system of equations below automatically.

$$ 1+\text{ln} p(x)-\lambda_0=0 $$ $$ \int_a^b p(x) dx=1 $$

My code is

Solve[1 - λ0 + Log[p[x]] == 0  &&  
  Integrate[p[x], {x, -Infinity, Infinity}] == 1 , {p[x], λ0} ]

But I get the error output,

Solve: This system cannot be solved with the methods available to Solve.

What should I do to make Mathematica able to solve the above system of equations?

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1 Answer 1

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Note: the mathematical description in the OP has a difference compared to the code provided, namely the limits of integration. I am following the TeX version of the equations provided.

Solve the first one

sltn1 = Assuming[Element[λ0, Reals], 
   Solve[1 - λ0 + Log[p[x]] == 0, p[x]]] // First

and use the solution to solve the other one

Solve[Integrate[p[x] /. sltn1, {x, a, b}] == 1, λ0] // First

Edit: ask yourself this. Since, we don't know what

$$ \begin{equation} \int^b_a dx ~ p(x) \end{equation} $$

is unless we specify $p(x)$, why should Mathematica evaluate it? This logic would lead you to break the problem into smaller parts that you can and the software can solve.

Edit: after the discussion in the comments with the author of the OP, many thanks for the input and verification, people who are using versions earlier than 13 should use

Assuming[{Element[λ0, Reals]},   Simplify[Solve[1 - λ0 + Log[p[x]] == 0, p[x]]]]

and the rest of the answer as it is.

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  • $\begingroup$ Thanks for your speedy answer. But I am still get the Solve: This system cannot be solved with the methods available to Solve. error. ibb.co/xmCGPCc $\endgroup$
    – benjaminchanming
    Jan 15 at 13:37
  • $\begingroup$ @Domen I am using 12.1.1.0 $\endgroup$
    – benjaminchanming
    Jan 15 at 13:40
  • $\begingroup$ @benjaminchanming try with a fresh kernel. Do Quit[] and then run the commands in the order I wrote them down and let me know. I hardly think that 12.1.1 will have troubles, so probably so previous definitions are messing up. $\endgroup$
    – bmf
    Jan 15 at 13:41
  • $\begingroup$ @benjaminchanming your notebook should look like this. Since your screenshot starts at input 11 I cannot know what happens in 11 lines of code I don't see :-) that's the reason for suggesting a clean kernel $\endgroup$
    – bmf
    Jan 15 at 13:45
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    $\begingroup$ @benjaminchanming perhaps in earlier versions you should wrap it inside a Simplify let's say. For example, does this Assuming[{Element[\[Lambda]0, Reals]}, Simplify[Solve[1 - \[Lambda]0 + Log[p[x]] == 0, p[x]]]] work? $\endgroup$
    – bmf
    Jan 15 at 14:23

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