In: Advanced Math
Problem 6. Let Pd (2, C) denote the vector space of C-polynomials in two variables, of degree ≤ d.
Define a linear map S : P2(2, C) → P2(C) by S(p) := p(z, z) (where z is a variable for the polynomials in P2(C)).
(a) Prove that S is surjective and that Skew2(2, C) ⊂ ker(S).
(b) Give an example of a polynomial in ker(S) \ Skew2(2, C). Hence write down a basis for ker(S).
(a) In one of the previous part we have seen that a basis of is . Also, we know that a basis of is , and a basis of is . Now, by definition of the linear map , we have
Therefore, the image of contains the basis of , showing that the image is itself. Hence, is surjective.
Since and , we conclude that the basis of the subspace is contained in the kernel. But then, the subspace itself must be contained in the kernel, because kernel is a subspace and hence is closed under linear span. Thus, .
(b) Since and , by rank-nullity theorem we get
But gives , and we already know that and is a basis for . Thus, if then a basis for would be .
Note that if then ; thus, satisfies , and a basis of is .