help-octave
[Top][All Lists]

## Newbie question solving lin sys

 From: John B. Thoo Subject: Newbie question solving lin sys Date: Fri, 27 Dec 2002 13:40:50 -0800

Hi. I'm very new to using Octave. I have two questions: 1. What am I doing wrong in the following? 2. How can I do it more elegantly?

I want to solve the system Ax = T where A is the 12x12 Vandermonde matrix

[ 1  0  0   ...  0    ]
[ 1  1  1   ...  1    ]
A = [ 1  2  2^2 ...  2^11 ]
[           ...       ]
[ 1 11 11^2 ... 11^11 ]

T = [ 22 28 31 39 46 53 56 55 49 42 33 27 ]'

To do this, I enter in Octave:

octave:51> A = fliplr(vander([0,1,2,3,4,5,6,7,8,9,10,11]));
octave:52> T = [22;28;31;39;46;53;56;55;49;42;33;27];
octave:53> A\T
ans =

2.2000e+01
1.2932e+02
-3.3001e+02
3.5674e+02
-2.1174e+02
7.7512e+01
-1.8446e+01
2.9092e+00
-3.0191e-01
1.9818e-02
-7.4570e-04
1.2250e-05

*But* when I go to check my answers, they seem to be off. To check my answers, I enter in Octave:

octave:54> s = 0;
octave:55> 22 + 129.32*s - 330.01*s**2 + 356.74*s**3 - 211.74*s**4 + 77.512*s**5 - 18.446*s**6 + 2.9092*s**7 - 0.30191*s**8 + 0.019818*s**9 - 0.0007457*s**10 + 0.00001225*s**11
ans = 22
octave:56> s = 1;
octave:57> 22 + 129.32*s - 330.01*s**2 + 356.74*s**3 - 211.74*s**4 + 77.512*s**5 - 18.446*s**6 + 2.9092*s**7 - 0.30191*s**8 + 0.019818*s**9 - 0.0007457*s**10 + 0.00001225*s**11
ans = 28.002
octave:58> s = 3;

and so on. The answers match well for the first few, but then they drift. To summarize, I get

s  |  answers from Octave  |  supposed to be (T)
---------------------------------------------------
0  |        22             |        22
1  |        28.002         |        28
2  |        31.017         |        31
3  |        38.996         |        39
4  |        45.498         |        46
5  |        50.205         |        53
6  |        46.456         |        56
7  |        30.029         |        55
8  |       - 5.5781        |        49
9  |       -62.227         |        42
10  |      -145.80          |        33
11  |      -245.29          |        27

---John.

-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------