Hi Team!
I'm using sympy to programmatically solve systems of equations, that are
essentially linear, but some equations contain Max or Min in some easy
manner. However, sympy.solve seems to throw a NotImplementedError in such
cases. This makes sense, if currently Min and Max are not supported by
sympy.solve.
That said, if I rewrite the equations to Piecwise, then it can solve the
system in many cases. But sometimes it throws an Invalid NaN comparison
somewhere in relational.py. See below for a minimal example that reproduces
the issue (system is x=Max(1, y), y=Max(2, x)).
I'm not quite sure if this is the intended behaviour or not, but I don't
know enough of the inner workings of sympy to identify the direct cause of
the NaN comparison exception. The solution I'm looking for here is x=y>=2.
Now if I only give one equation (say, x=Max(1,x)), then the Piecewise
version throws me a NotImplementedError, but also tells me that the
solution is likely containing the region x>1, which is helpful.
Finally, solveset corretly identifies the interval solution for x=Max(1,x)
for reals, but I don't know is solveset can be directly called somehow on a
system or not.
So my questions are:
1) Is the expected behaviour for sympy.solve for the Piecewise case to
throw a NaN comparison exception, or is this a sign of some underlying
issue in sympy.solve for Piecewise functions?
2) Is sympy.solve expected to rewrite Min and Max to Piecewise at some
point in the future? I found some piece of code in
solvers/solvers.py:944-957 which does some sort of rewriting for Abs, but
didn't find any for Min or Max.
3) From some reading on the sympy docs page, I figured solveset is expected
to be the future solver. Is there a direct way I can apply solveset on a
system, rather than on one equation?
4) If the answer to 3) is yes, is there also a way which provides a stable
output that I can use in a programmatic way?
Thanks for the help!
Gábor
#######################################
## Minimal working example reproducing the error: ##
#######################################
import sympy
x, y = sympy.symbols('x y', real=True)
sys = [sympy.Eq(x, sympy.Max(y, 1)), sympy.Eq(y, sympy.Max(2, x))]
variables = (x, y)
sympy.solve(sys, variables)
new_sys = [eq.rewrite(sympy.Piecewise) for eq in sys]
sympy.solve(new_sys, variables)
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