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Strange matrix product

asked 2019-02-25 07:50:00 -0600

SteveS gravatar image

updated 2019-05-05 17:41:42 -0600

slelievre gravatar image

When I enter

A = matrix([[1, 2], [-1, 0], [1, 1]])
B = matrix([[0, 4], [1, -1], [1, 2]])
A*B

I get an error message, as expected.

But when I enter

A = matrix(QQ, [[1, 2], [-1, 0], [1, 1]])
B = matrix(QQ, [[0, 4], [1, -1], [1, 2]])
A*B

I get the output

[ 1  0]
[ 1 -2]
[ 1  3]

However, if I try

A = matrix(QQ, [[1, 2], [-1, 0], [1, 1]])
B = matrix(QQ, [[0, 4], [1, -1], [1, 2], [2, 2]])
A*B

I get an error message.

In all of the examples I try, Sage will give some output for a matrix product of two matrices of the same size as long as the field QQ is specified for at least one of the matrices. I’ve searched online and haven’t found anything to help me understand this. This will be problematic for my students who might draw the conclusion that the standard matrix product is defined when it really isn't. Can anyone help me understand this?

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answered 2019-02-25 13:20:05 -0600

slelievre gravatar image

updated 2019-05-05 17:41:07 -0600

Thanks for catching this! Indeed an oversight.

Fixing this is now tracked at


Update (2019-05-06). The fix was merged in SageMath 8.7.beta7 and is in particular available in SageMath 8.7, released 2019-03-23.

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Asked: 2019-02-25 07:50:00 -0600

Seen: 95 times

Last updated: May 05