On Tue, 23 Feb 2021 at 21:06, Bruno Nicenboim <bruno.nicenb...@gmail.com> wrote:
>
> On Tuesday, February 23, 2021 at 9:53:03 PM UTC+1 Oscar wrote:
>>
>> On Tue, 23 Feb 2021 at 18:08, Bruno Nicenboim <bruno.n...@gmail.com> wrote:
>> >
>> > Hi,
>> > This is my first email. I'm starting to use sympy, which I find
>> > fascinating, and I have some questions that couldn't be answered after
>> > going through the documentation and stackoverflow website.
>>
>> Hi Bruno and welcome!
>>
>> Feel free to ask questions here. I didn't see any actual questions in
>> your email though...
>>
> Yeah, I just wanted to check that the mail goes through :)
>
> I'm cross posting with stackoverflow where I didn't get any answer:
> https://stackoverflow.com/questions/66200069/restricting-the-domain-of-the-solution-of-solveset-with-sympy
>
> I'm trying to impose a constraint on the variable that I want to solve for
> with sympy. (That is setting a domain that needs to be respected). And I want
> to use `solveset` (I've seen other answers that use `solve`).
>
> Say I have the following `K_p` and I want to solve `K_p - x = 0` for `t`:
>
> ```
> from sympy import *
> psi, mu_1, mu_2, tau, t, x = symbols("psi, mu_1, mu_2, tau, t, x", positive =
> True, real = True)
> K_p =(mu_1*mu_2*psi*t**2*tau**4 - 2*mu_1*mu_2*t*tau**2 - mu_1*psi*t*tau**2 +
> mu_1 - mu_2*psi*t*tau**2 + mu_2 + psi)/(mu_1*mu_2*t**2*tau**4 - mu_1*t*tau**2
> - mu_2*t*tau**2 + 1)
> ```
>
> How do I impose the following constraints:
> `t < 1/(mu_1 * tau**2)` and `t < 1/(mu_2 * tau**2)`
>
> If I do this I have an extra solution that shouldn't be there:
> ```
> s_x = solveset(K_p - x, t, domain=S.Reals)
> ```
I don't think your constraints are enough to determine what the
correct solution should be. For example if we have the values:
{mu_1:1, mu_2:2, tau:2, psi:1, x:2}
Then neither solution satisfies the constraint:
In [218]: rep = {mu_1:1, mu_2:2, tau:2, psi:1, x:S.Half}
In [219]: conditions = And(t < 1/(mu_1 * tau**2), t < 1/(mu_2 * tau**2))
In [220]: eq = K_p - x
In [221]: solve(eq.subs(rep))
Out[221]:
⎡11 √65 √65 11⎤
⎢── - ───, ─── + ──⎥
⎣16 16 16 16⎦
In [222]: r1, r2 = solve(eq.subs(rep))
In [223]: conditions.subs(rep).subs(t, r1)
Out[223]: False
In [224]: conditions.subs(rep).subs(t, r2)
Out[224]: False
--
Oscar
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