# change of basis coordinates

• Dec 19th 2012, 06:25 PM
sfspitfire23
change of basis coordinates
Hi all,
I have an orthogonal basis consisting of the following vectors in C^4 -

[0,1,0,-1] , [1,0,0,0] , [0,0,i,0]

Now, I have the vector [1, -i, 0, i] in standard coordinates. I wish to express this vector with respect to my orthogonal basis.

So, I solve c1 [0,1,0,-1] + c2 [1,0,0,0] + c3 [0,0,0,0] + c4[0,0,i,0] = [1, -i, 0, i].

[c1,c2,c3,c4] will be the vector wrt my othogonal basis.

Is this correct?

Thanks.
• Dec 19th 2012, 08:02 PM
chiro
Re: change of basis coordinates
Hey sfspitfire23.

Have you set up your augmented system? (Hint: Setup the augmented system [A | b] and solve for [I | x])
• Dec 19th 2012, 08:35 PM
Deveno
Re: change of basis coordinates
IF (1,-i,0,i) is in the span of your three vectors, then you only need to solve:

c1(0,1,0,-1) + c2(1,0,0,0) + c3(0,0,i,0) = (1,-i,0,i) for c1,c2 and c3 (adding a 0-vector to a basis is a waste of time).

inspection of the first coordinate shows c2 = 1. inspection of the fourth coordinate shows c1 = -i.

finally, since (-i)(0,1,0,-1) + (1)(1,0,0,0) = (0,-i,0,i) + (1,0,0,0) = (1,-i,0,i), we conclude c3 = 0, so our coordinates in this basis are [-i,1,0].

note you only specified THREE basis vectors, so we only have THREE coordinates (that is, if our basis only has 3 elements, the span of this basis is a 3-dimensional subspace of C4).
• Dec 20th 2012, 05:01 AM
sfspitfire23
Re: change of basis coordinates
Deveno,

Thanks. Is including the zero vector technically wrong? Or is it just redundant?
• Dec 20th 2012, 01:39 PM
Deveno
Re: change of basis coordinates
including a 0 vector makes a set linearly dependent.