Who is the encoder of jordan_form in SageMath?

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To complement @FrédéricC 's answer, here is how to find that the relevant source file is indeed src/sage/matrix/matrix2.pyx: in a SageMath session, run

sage: m = matrix(1)
sage: m.jordan_form?

At the end of the returned documentation, you can see src/sage/matrix/matrix2.pyx in the File: field. Then you can browse the github directory to that file, as indicated by @FrédéricC.

Alternatively, you can run git blame on your computer: first open a terminal and go to the subdirectory src/sage/matrix of your SageMath root directory (the latter is returned by sage: !echo $SAGE_ROOT in a SageMath terminal). Then determine the line at which the definition of the method jordan_form starts by running

grep -n "def jordan_form" matrix2.pyx

For SageMath 9.7, the output should be:

10823:    def jordan_form(self, base_ring=None, sparse=False, subdivide=True, transformation=False, eigenvalues=None, check_input=True):

Hence the line number is 10823 and you may run

git blame -L 10823 matrix2.pyx

to see the list of contributors to the method jordan_form.