Solving Det(A) in an Invertible 5x5 Matrix with Adjoints

[daniel_i_l]

Homework Statement

a)A is an invertible 5x5 matrix with complex elements. Find all the values of det(A) where adj(adj(A)) = A. Use the definition of the adjoint (transpose of cofactor matrix)

b) u1,u2,u3 are linearly independent vectors in V. We define:
v1=u1+u2+u3, v2=u1-u2+u3, v3=-u1+3u3-u3
Does Sp{u1,u2,u3}=Sp{v1,v2.v3} ?

Homework Equations

a) The definition of adj(A)
b) If A and B are subsets of space V then Sp(A) = SP(B) iff A is in Sp(B) and B is in Sp(A).

The Attempt at a Solution

a) I tried to use the definition of the adjoint to find the adjoint of adj(A) but it quickly got so complicated that I couldn't see how to calculate det(A) from it. Is there a way to use the fact that A is a 5x5 matrix to simplify things?
b)I think that the answer is yes because obviously {v1,v2,v3} is in the span of {u1,u2,u3} because of their definition. And by solving the equation:
[v1|v2|v3][x]=[un] for all 0<n<=3 I can show that every u is a linear combination of v's and so {u1,u2,u3} is in Sp{v1,v2,v3}. Is that right?
Thanks.

 

[matt grime]

When you say you used the definition of adj(A), do you mearn you used the definition, or the formula? adj(A), is the (unique for det=/=0) matrix satisfying A*adj(A)= det(A)*Id.

 

[daniel_i_l]

I mean the formula: $$[adjA]_{ij} = (-1)^{i+j}*M_{ji}$$
Where $$M_{ji}$$ is the minor (ji). (by minor (j,i) I mean the det of the matrix obtained by taking away the jth row and the ith coloum)

 

[matt grime]

yes, I know that what was you used. I didn't ask for my benefit.

 

[HallsofIvy]

Now, what relation does det(A) have to $$[adjA]_{ij} = (-1)^{i+j}*M_{ji}$$?

As for B, you were told that u1, u2, and u3 are linearly independent. That means they are a basis for their span which must be of dimension 3. The real question is "Are v1, v2, and v3 linearly independent?"