I think you are right this is a bug.
>>> x, y = symbols('x y', commutative=False)
>>> M1 = x*eye(2)
>>> y * (x*M1)
Matrix( [ [x*y, 0], [x*y, 0] ] )
# i think the output should be Matrix( [ [y*x, 0], [0, y*x] ] )
On Tuesday, May 12, 2015 at 1:52:01 AM UTC+5:30, Carsten Knoll wrote:
>
> Hello,
>
> I wonder about the following behavior
>
>
> In [1]: import sympy as sp
>
> In [2]: x, y = sp.symbols('x, y', commutative = False)
>
> In [3]: M1 = x*sp.eye(2)
>
> In [4]: M2 = y*x*sp.eye(2)
>
> In [5]: y*M1-M2
> Out[5]:
> Matrix([
> [x*y - y*x, 0],
> [ 0, x*y - y*x]])
>
>
>
> I would have expected that the two matrices are equal.
>
> However, multiplying y from the right to M1 results in products where y
> is on the left side:
>
> In [6]: y*M1
> Out[6]:
> Matrix([
> [x*y, 0],
> [ 0, x*y]])
>
>
> For reference two other results:
>
> In [7]: M2
> Out[7]:
> Matrix([
> [y*x, 0],
> [ 0, y*x]])
>
> In [8]: M1*y
> Out[8]:
> Matrix([
> [x*y, 0],
> [ 0, x*y]])
>
>
> Questions:
>
> 1. Is this behavior intended?
> 2. Is there any (easy) way to achieve my intended behavior, i.e.
> respecting multiplication order when using matrix multiplication.
>
>
> Thanks,
> Carsten.
>
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