# Subsets of the complex space

1. Apr 15, 2007

### daniel_i_l

1. The problem statement, all variables and given/known data
Let:
$$\alpha \in C_{4}[x]$$ (the space of all 4-deg complex ploymonials)
We'll define:
$$U = Sp( \{\ x^3 - 2i \alpha x, x^2 +1 \} ) \\ W = Sp( \{\ x^3 + ix, (1-i)x^2 - \alpha x \})$$
as subspaces of $$C_{4}[x]$$
a) find all values of alpha so that:
$$W \cap U \neq \{ 0 \}$$
b) does:
$$C_{4}[x] = W \oplus U$$
when alpha = 0?

2. Relevant equations
If $$C_{4}[x] = W+U$$ and $$W \cap U = {0}$$
then $$C_{4}[x] = W \oplus U$$

3. The attempt at a solution
I'm not sure how to approch this question. Do I find all the cases where
$$W \cap U = {0}$$ and then write the answer as all the other cases? To do that do I first find all the alphas where u = w for some u in U and for some w in W and then check which one of those forces both u and w to be 0?
And for b, if the answer in a isn't 0 then the answer is no right?
Thanks.
EDIT: Where did all the curly brackets go?

Last edited by a moderator: Apr 15, 2007
2. Apr 15, 2007

### HallsofIvy

For a, I see no reason not to do it directly: any vector in U can be written $m(x^3- 2i\alpha x)+ n(x2+1) and any vector in V can be written [tex]a(x3+ ix)+ b((1+i)x2- \alpha x). Set those equal to each other. For what values of [itex]\alpha$ can you NOT find m,n,a, and b that will work?

As for b, C4[x] includes x4 but neither U nor V does.