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Unexpected result of equation solving in Wolfram|Alpha

Posted 2 years ago

Good day for all!

There is an incorrect result for equation solving
https://www.wolframalpha.com/input?i=solve%28%288%5Ex-2%5Ex%29%2F%286%5Ex-3%5Ex%29%3D%3D2%2Cx%29
over the reals. There must be only one corrent solution x=1, but WA and WM (Reduce / Solve) also provides x=0 which is incorrect.
Why doesn't WA/WM check the validness of its results?

POSTED BY: VolMike VolMike
7 Replies

You are correct, it is only removing the algebraic ones, that is, those that arise from rational functions. It is not removing more general ones from meromorphic functions.

POSTED BY: Daniel Lichtblau

If Mathematica removed all removable singularities I would expect this to give x->0:

Solve[Sin[x]/x == 1 && Abs[x] < 1, x]

just as I get for this:

Solve[Sinc[x] == 1 && Abs[x] < 1, x]

Maybe the full story is more complicated.

POSTED BY: Gianluca Gorni

I didn't claim solutions at singularities would be removed. I stated that the singularities themselves would be removed, that is, not be regarded as singularities (that's what it means to remove a removable singularity). Stated in terms of the most recent example, in the field of rational functions (x^2-1)/(x-1) is equivalent to (x+1).

This is not regarded as broken by the way. Wolfram|Alpha is more in tune with what is taught in high schools, and will attempt to remove singular solutions. So that particular example will have an empty solution set. Different philosophy.

POSTED BY: Daniel Lichtblau

Well, if I understood your right, it's not true: as of 13.0.1 W|M Solve doesn't remove singular solutions. Look at the example above with Solve[(x^2 - 1)/(x - 1) == 2, x].

Or did you mean it will be fixed in the future versions of W|M?

POSTED BY: VolMike VolMike
POSTED BY: Daniel Lichtblau

Thanks for your answer!

But nevertheless, WM/WA should check back the original equation automatically without extra functions such as FunctionDomain when the original and simplified equations are not equivalent.

POSTED BY: VolMike VolMike
POSTED BY: Gianluca Gorni
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