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Correct and incorrect answer from linsolve

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Oddur Bjarnason
Oddur Bjarnason on 14 Oct 2016
Edited: Walter Roberson on 17 Oct 2016
The following gives a correct answer:
A=[-2/5,1/5;2/5,-1/5;1,1]
B=[0;0;1]
linsolve(A,B)
ans =0.3333 0.6667
The following, however, gives an incorrect answer:
C=[-0.2,0.3;0.25,-0.3;1,1]
D=[0;0;1]
C\D
ans = 0.5699 0.4297
The correct answer, found by substitution, is 0.6000 0.4000
How can I get a correct answer by linsolve?

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Answers (1)

Walter Roberson
Walter Roberson on 14 Oct 2016
x = sym('x',[2 1]);
C*x
ans =
(3*x2)/10 - x1/5
x1/4 - (3*x2)/10
x1 + x2
You have a system of 3 equations in 2 unknowns. It is overdetermined, and might not have any exact solution. The \ operation will do a least-squared fit to find an answer that is least bad in some sense.
Solving (3*x2)/10 - x1/5 = 0 for x1 gives x1 = (3*x2)/2. Substituting that back into C*x gives
0
(3*x2)/40
(5*x2)/2
solving (3*x2)/40 = 0 for x2 gives x2 = 0. Substituting that back gives (5*0)/2 = 1 which is 0 = 1 which has no solution.
>> C*[0.6;0.4]
ans =
0
0.03
1
So 0.6 0.4 is not a solution after-all.
>> sum((D-C*(C\D)).^2)
ans =
0.000407074042245239
>> sum((D-C*[0.6;0.4]).^2)
ans =
0.0009
so the solution found by C\D gives less of an error than [0.6 0.4] does.

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Oddur Bjarnason
Oddur Bjarnason on 17 Oct 2016
Dear Walter and Steven
Thank you for your answer/comments.
A Markov chain has the initial state vector S0= [0.25,0.75] and the transition matrix T=[0.8,0.2;0.3,0.7].
The stationary state vector is approached by [0.25,0.75]*[0.8,0.2;0.3,0.7]^N if N is sufficiently high. For N is = 100 the stationary state vector is [0.6,0.4].
Let the stationary state vector be SS and let SS=[SS1,SS2]. The vector can be found by solving the following equation:
SS*T=SS
or
[SS1,SS2]*[0.8,0.2;0.3,0.7]=[SS1,SS2]
This yields the following system of linear equations:
0.8*SS1+0.3*SS2=SS1
0.25*SS1+0.7*SS2=SS2
SS1+SS2=1
This equation can be solved by substitution and the stationary state vector SS = [SS1,SS2] = [0.6,0.4].
It should also be possible to solve the system of equations by linsolve. Before trying to do that SS1 must be subtracted from both sides of the first equation and SS2 from both sides of the second equation, yielding the following equations:
-0.2*SS1+0.3*SS2=0
0.25*SS1-0.3*SS2=0
SS1+SS2=1
The coefficient matrix is
A=[-0.2,0.3;0.25,-0.3]
and the vector is
B=[0;0;1]
The solution by linsolve is
linsolve(A,B) = [0.5699,0.4279] Which is approximately "correct".
Can you help me to understand why [0.6,0.4] should not be a solution?
Steven asked what I was trying to do.
I am interested in developing mathematical models of migration. One of the methods I should like to use is Markov chains. I thought that it might be good to use MATLAB seing that it is based on matrices. I therefore got MATLAB,started learning it and began to try to use it for studying Markov chains. As you see I am still crawling.
Another reason, quite unrelated, for getting MATLAB, was that I bought a book called Cosmology with Matlab and thought that I should like to have access to MATLAB while reading it.
Steven Lord
Steven Lord on 17 Oct 2016
Now the cause of the problem is clear. You wrote:
This yields the following system of linear equations:
0.8*SS1+0.3*SS2=SS1 0.25*SS1+0.7*SS2=SS2 SS1+SS2=1
You have a typo in your second equation. You wrote 0.25 instead of 0.2. So linsolve is solving the problem you told it to solve, not the problem you wanted it to solve. When I write the correct coefficient matrix, both linsolve and backslash (\) return the answer you expected.
A = [0.8-1 0.3; 0.2 0.7-1; 1 1];
b = [0; 0; 1];
x1 = linsolve(A, b)
x2 = A\b
Since you're interested in Markov chains you may find the PageRank and Markov Chains section of the chapter on Linear Equations in Cleve's Numerical Computing in MATLAB interesting and informative.
Oddur Bjarnason
Oddur Bjarnason on 17 Oct 2016
Thank you Steven, When I was filling in the coefficient Matrix I read 0.2S1 to be 0.25. So you are right - it was a typo.

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