Warning: Matrix is singular to working precision

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?

Answer 1

[Chunru]

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]

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 .