Unable to find explicit solution

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Tiku 2021-4-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/790124-unable-to-find-explicit-solution

评论： Tiku 2021-4-4

采纳的回答： Matt J

在 MATLAB Online 中打开

Matlab says "Unable to find explicit solution" when trying to solve following non linear equation

-log(y) = psi(1/2+ x/y) - psi(1/2)

Here psi is digamma function

What is wrong with my code, below?

I am using matlab version R2021a

syms x y 
eqn = psi(1/2 + x/y)- psi(1/2)+ log(y)==0;
soly = solve(eqn,y)
Warning:
Unable to find explicit solution. For options, see help. 
> In sym/solve (line 317)
soly =
 
Empty sym: 0-by-1

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Matt J 2021-4-1

It seems very plausible to me that an explicit solution does not exist. Do you have reason to think otherwise?

Tiku 2021-4-1

Thank you for the reply.

I see explicit solution doesn't exist.

That means in order to plot y vs x I should assigned the value of x and solve for y right?

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采纳的回答

Matt J 2021-4-1

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/790124-unable-to-find-explicit-solution#answer_664614

You can use fimplicit.

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Matt J 2021-4-2

We've no reason to trust the paper more than Matlab...

Tiku 2021-4-4

I totally agree Matlab calulation is right. Actually after the plot I have to use some physics to ignore certain parts of the plot so at this stage both Matlab and paper is right.

So I really apppreciate your help and thank you.

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