Status: Accepted
Owner: jochen.voss
Labels: Type-Defect Priority-Medium
New issue 1421 by jochen.voss: QRdecomposition fails for some matrices
http://code.google.com/p/sympy/issues/detail?id=1421
Hello,
on the current git version of sympy I got a strange failure of the
Matrix.QRdecomposition method:
===========================================================================
In [1]: A = Matrix([[1,1],[1,1.01]])
In [2]: A.QRdecomposition()
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
/Users/voss/sympy/<ipython console> in <module>()
/Users/voss/sympy/sympy/matrices/matrices.py in QRdecomposition(self)
966 R[j,j] = tmp.norm()
967 Q[:,j] = tmp / R[j,j]
--> 968 assert Q[:,j].norm() == 1
969 for i in range(j):
970 R[i,j] = Q[:,i].dot(self[:,j])
AssertionError:
===========================================================================
This seems not to be a problem with the conditioning of A, since it works
in the following case (which has a worse conditioning):
===========================================================================
In [3]: A = Matrix([[1,1],[1,1.001]])
In [4]: A.QRdecomposition()
Out[4]:
⎛⎡ ⎽⎽⎽ ⎤ ⎞
⎜⎢╲╱ 2 ⎥ ⎡ ⎽⎽⎽ ⎽⎽⎽ ⎤⎟
⎜⎢───── -0.707106781186548⎥, ⎢╲╱ 2 1.0005⋅╲╱ 2 ⎥⎟
⎜⎢ 2 ⎥ ⎢ ⎥⎟
⎜⎢ ⎥ ⎣ 0 0.00070710678118647⎦⎟
⎜⎢ ⎽⎽⎽ ⎥ ⎟
⎜⎢╲╱ 2 ⎥ ⎟
⎜⎢───── 0.707106781186548 ⎥ ⎟
⎝⎣ 2 ⎦ ⎠
===========================================================================
I hope this helps,
Jochen
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