How to get the left matrix that was used to obtain reduced row echelon form?

asked 2018-09-10 17:25:45 +0100

Jules
11 ●1 ●2

Hi all.

I have a matrix A and would like to determine a matrix E such that E*A = R, where R is the reduced row echelon form of the matrix A. I just know the command R = A.rref(). But I don't know how to get E (my stupid solution is to compute the inverse of the pivot columns of A ...). Is there e.g. an argument such that A.rref() also outputs E?

Thanks a lot, Julian

answered 2018-09-10 18:31:53 +0100

slelievre
17684 ●22 ●160 ●349 http://carva.org/samue...

You can use the echelon_form method with transformation=True.

Example:

sage: A = Matrix([[1, 2, 3], [3, 2, 1], [1, 1, 1]])
sage: A
[1 2 3]
[3 2 1]
[1 1 1]
sage: A.echelon_form(transformation=True)
(
[ 1  0 -1]  [ 0  1 -2]
[ 0  1  2]  [ 0 -1  3]
[ 0  0  0], [ 1  1 -4]
)

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Thanks for your quick answer. However, it does not really what I would like to have :-/ Am I doing something wrong or does It not work if A is over GF(q). (gives A.echelon_form(transformation=True) always reduced row echelon form or just echelon form?)

Thanks :)

Jules ( 2018-09-11 16:22:27 +0100 )edit

There are two bugs in Sage regarding this:

The method for reduced row echelon form, rref, calls the method echelon_form, and is supposed to pass on the arguments it is given, such as transformation=True, but it does not.
The method echelon_form itself does not always honor the request to return the transformation (i.e., the argument transformation=True).

See

Sage Trac ticket 23715: most matrices ignore transformation=True in echelon_form.

slelievre ( 2018-09-12 00:42:22 +0100 )edit

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