
Ublas : 
Subject: [ublas] Help with lu_factorize
From: Marco Guazzone (marco.guazzone_at_[hidden])
Date: 20090915 11:46:15
Hello!
I don't understand how the function lu_factorize(A) works (note: only
the version with one parameter, which I think is for LU without
pivoting).
I didn't find any documentation of this function so I've decided to
experiment a bit with it.
I've attached an example.
The input matrix is:
[snip]
Input matrix
matrix:
[4,4]((0.55595,0.27469,0.540605,0.798938),(0.108929,0.830123,0.891726,0.895283),(0.948014,0.973234,0.216504,0.883152),(0.023787,0.675382,0.231751,0.450332))
[/snip]
In this example I use both lu_factorize(A) and lu_factorize(A,P)
(i.e.,with the permutation matrix).
The last one works perfectly. After call it, A is what I expected
(also verified with Mathematica and Octave).
[snip]
LU *with* permutation matrix
res: 0
matrix:
[4,4]((0.948014,0.973234,0.216504,0.883152),(0.114902,0.718296,0.866849,0.793807),(0.586436,0.412156,0.770916,0.608198),(0.0250914,0.906259,0.725463,0.150003))
[/snip]
The first one instead gives a result that I'm not able to understand.
[snip]
LU *without* permutation matrix
res: 0
matrix:
[4,4]((0.55595,0.27469,0.540605,0.798938),(0.195933,0.776302,0.785804,0.738745),(1.70521,0.650299,1.21635,0.959614),(0.0427862,0.854859,0.380754,0.150003))
[/snip]
NOTE: I'm using Boost 1.40 and GCC 4.4.1. I've compiled with:
$ g++ o lu Wall Wextra pedantic ansi lu.cpp
Can you give me an help, please?
Best,
 Marco