# Revision history [back]

Thanks for reporting this bug here and on Sage Trac:

To analyse the error coming from:

sage: M = Matrix([0], ring=GF(4))
sage: M
[0]
sage: M.inverse()
[1]


we can check the documentation and the source code for the inverse method of M:

sage: M.inverse?
sage: M.inverse??


and we see that it calls ~M. In Sage, ~M calls M.__invert__().

The source code for the __invert__ method revealed by

sage: M.__invert__??


involves mzed_invert_newton_john from m4rie.

The source code for that function is at

• https://github.com/malb/m4rie/blob/9f39ae950a733bd1e9ced74f59d9b594f5a7296c/src/newton_john.c#L508

and it echelonizes the augmented matrix to compute the inverse --- a fine thing to do for an invertible matrix.

There is now a fix at the Sage Trac ticket. After the fix, we first check whether the matrix has full rank; if not, we raise an error; if yes, we compute the inverse by echelonizing the augmented matrix.