sympy/sympy — pull request #650 by asmeurer (2011-10-12 04:43:13+00:00)
Title: Use PurePoly for Matrix.charpoly, and let it work without specifying a Sy

Use PurePoly for Matrix.charpoly, and let it work without specifying a Symbol

I used Symbol('lambda') as the default Symbol, since this is the common
name used for this variable.