Comment #4 on issue 2009 by mattpap: Document why solve((x-y,y),x) is None
http://code.google.com/p/sympy/issues/detail?id=2009
I disagree with comment #1. Currently solve([x - y, y], x) returns None but
solve(x - y, x) returns [y]. In SymPy, solve([x - y, y], x) means
solve([Eq(x - y, 0), Eq(y, 0)], x), where y is a free variable. This way we
let y to be zero and solve a single linear equation. This is equivalent to
writing:
y = 0
solve(x - y, x)
[0]
So, solve([x - y, y], x) should return [0] (or any equivalent data
structure, e.g. {x: 0}, or event better some SolutionSet class). btw.
Mathematica can do this and returns {x -> 0}}.
This approach could be even generalized to allow things like:
solve([x**3 - 1, x in QQ], x)
{x: 1}
assuming that x in QQ would return (for example) Element(x, QQ) instance.
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