# [Numpy-discussion] question on differential results in windows and debian

meng at are.berkeley.edu meng at are.berkeley.edu
Tue Oct 11 14:14:22 EDT 2005

```Hi there-

I have a matrix X, calculate cX=matrixmultiply(transpose(X), X) and finally
invert it by icX = linear_algebra.inverse(cX)
cX is kind of ill-conditioned, which caused the icX differs a lot between
windows and debian, after a tiny difference on cX across platforms. Here
are the results:
In windows:
>>> x.min()
-0.51187150837988826
>>> x.max()
0.81517690875232773
>>> cX = matrixmultiply(transpose(x), x)
>>> cX.min()
-666.96857541904126
>>> cX.max()
1073.3945530725441
>>> import numarray.linear_algebra as la
>>> icX = la.inverse(cX)
>>> icX.min()
-3287030277.3580685
>>> icX.max()
0.0

In debian:
>>> x.min()
-0.51187150837988826
>>> x.max()
0.81517690875232773
>>> cX = matrixmultiply(transpose(x), x)
>>> cX.min()
-666.96857541898294
>>> cX.max()
1073.3945530726264
>>> import numarray.linear_algebra as la
>>> icX = la.inverse(cX)
>>> icX.min()
0.0
>>> icX.max()
84778028554.337051

I don't understand why a multiplication of a matrix and its transpose will
create a tiny difference across windows and debian, which could be
disastrous when inverting it.
Please help me. Thanks a lot for your time! BTW, I am using Python2.4 and
numarray 1.3.2 in both machines.

Best,
Xiangyi

Xiangyi Meng
Department of Agricultural and Resource Economics
Room 303, Giannini Hall #3310
University of California, Berkeley
Berkeley, CA 94720-3310
Tel:  (510) 643-4124
Fax: (510) 643-8911
Email: meng at are.berkeley.edu

```

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