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I frequently have trouble trying to place limits on Solve or Reduce using assumptions and domains. For example, this code:

Assuming[x > 0 && x < 1, Solve[(1 - (1 - x)^3)^3 == x, x, Reals]]

Produces results both with complex number, and with x outside the range specified. The exact output is long (and includes the desired results), but a subset is pasted below. What is going on? $$\left\{\{x\to 0\},\{x\to 1\},\left\{x\to 1-\frac{\left(1-i \sqrt{3}\right) \left(\frac{1}{2} \left(-9+\sqrt{93}\right)\right)^{1/3}}{2 3^{2/3}}+\frac{1+i \sqrt{3}}{2^{2/3} \left(3 \left(-9+\sqrt{93}\right)\right)^{1/3}}\right\}\right\}$$

OctaviaQ
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1 Answers1

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Solve[{(1 - (1 - x)^3)^3 == x, 0 < x < 1}, x, Reals] works fine in Mathematica 8... Reduce[{(1 - (1 - x)^3)^3 == x, 0 < x < 1}, x, Reals] is pretty much equivalent.

J. M. ain't a mathematician
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  • Hmmm.. I am on 7 right now, but I believe I had the same problem on my home computer, which has 8 (for students). I'll double check when home. But even if it is just for 7, I still don't get why it doesn't work. The code we entered was exactly the same. – OctaviaQ Sep 18 '11 at 16:37
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    Well, Assuming[] does not seem to be suited for use with Solve[] or Reduce[]; better to input the constraints along with the equations for these. – J. M. ain't a mathematician Sep 18 '11 at 16:51
  • Hmmm... ok when I use reduce, and I do it the way you suggest, it works. Solve still gives me complex numbers. But, this is better, thanks! – OctaviaQ Sep 18 '11 at 16:53
  • Still have the problem in v8, but only sporatically. I guess I'll just assume it's a bug. – OctaviaQ Sep 25 '11 at 18:15
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    @JandR: As a point of note, Solve changed between v.7 and v.8. In v.7 the third param was for variables to eliminate, and in v.8 it is for the domain. Also, according to the docs, Assuming only affects those functions that have an Assumptions option. So, you have to include your assumptions as extra equations passed to Solve and Reduce, as J.M. shows. – rcollyer Oct 09 '11 at 17:55
  • I would like to disagree. Your first example does NOT work fine in Mathematica 8: In[16]:= Solve[{(1 - (1 - x)^3)^3 == x, 0 < x < 1}, x, Reals]

    During evaluation of In[16]:= Solve::eqf: x<1 is not a well-formed equation. >>

     – András Aszódi Feb 23 '14 at 06:45