Status: Accepted
Owner: [email protected]
Labels: Type-Defect Priority-Medium Solvers
New issue 2750 by [email protected]: solve could do a better job with over
determined systems
http://code.google.com/p/sympy/issues/detail?id=2750
I think this first item is actually a bug, considering the current state of
solve:
In [77]: solve(Eq(I3 - I4, -5*I3/2 + Q2/2))
Out[77]:
⎡⎧ 2⋅I₄ Q₂⎫⎤
⎢⎨I₃: ──── + ──⎬⎥
⎣⎩ 7 7 ⎭⎦
Why did it pick I3 to solve for (other than that it read my mind, because
that was actually the one I wanted)?
More generally, I'd like to be able to tell solve to give me any valid
expression for some variable based on the system, to help me eliminate
variables by substitution:
In [78]: print system
(I1 - I2 - I3, I3 - I4 - I5, I4 + I5 - I6, -I1 + I2 + I6, -2*I1 - 2*I3 -
2*I5 - 3*I6 - dI1/2 + 12, -I4 + dQ4, -I2 + dQ2, 2*I3 + 2*I5 + 3*I6 - Q2, I4
- 2*I5 + 2*Q4 + dI4)
In [79]: solve(system, I3)
Or better, it would be nice if I could just tell solve that I want
everything in terms of I1, I4, Q2, Q4, dI1, and dI4, and it would just do
it, eliminating the other four variables. If the system is completely
linear (as it is here), this is just a matter of getting the null space in
terms of the right free variables. For non-linear systems, you can still
sometimes do it by substitution.
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