# Warning: Matrix is singular to working precision

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oun alsharif on 14 Nov 2021
Edited: Chunru on 14 Nov 2021
a=[1, 0, 0, 0, -0.25, 0; 0, 1, 0, -0.25, 0, -0.25; 1, 0, 0, 0, -0.25, 0; 0, -0.25 0, 1, 0, 0; -0.25, 0, -0.25, 0, 1, 0; 0,-0.25, 0, 0, 0, 1];
b=[6.25,4.45,6.8,5.5,3.87,6.45];
c=b/a;
Warning: Matrix is singular to working precision.
i dont know why?

Chunru on 14 Nov 2021
Edited: Chunru on 14 Nov 2021
Make sure your matrix "a" is full rank (rank(a) should be 6 instead of 5).
a=[1, 0, 0, 0, -0.25, 0; 0, 1, 0, -0.25, 0, -0.25; 1, 0, 0, 0, -0.25, 0; 0, -0.25 0, 1, 0, 0; -0.25, 0, -0.25, 0, 1, 0; 0,-0.25, 0, 0, 0, 1]
a = 6×6
1.0000 0 0 0 -0.2500 0 0 1.0000 0 -0.2500 0 -0.2500 1.0000 0 0 0 -0.2500 0 0 -0.2500 0 1.0000 0 0 -0.2500 0 -0.2500 0 1.0000 0 0 -0.2500 0 0 0 1.0000
rank(a)
ans = 5
b=[6.25,4.45,6.8,5.5,3.87,6.45]
b = 1×6
6.2500 4.4500 6.8000 5.5000 3.8700 6.4500
c=b/a;
Warning: Matrix is singular to working precision.
Walter Roberson on 14 Nov 2021
The important point here is the fact that a has rank 5, so the matrix is singular.
Notice that your first and third row are the same. Any matrix which has identical rows will be singular .