Paper 4, Section II, G

Linear Algebra
Part IB, 2014

Let VV be a real vector space. What is the dual VV^{*} of V?V ? If e1,,ene_{1}, \ldots, e_{n} is a basis for VV, define the dual basis e1,,ene_{1}^{*}, \ldots, e_{n}^{*} for VV^{*}, and show that it is indeed a basis for VV^{*}.

[No result about dimensions of dual spaces may be assumed.]

For a subspace UU of VV, what is the annihilator of UU ? If VV is nn-dimensional, how does the dimension of the annihilator of UU relate to the dimension of UU ?

Let α:VW\alpha: V \rightarrow W be a linear map between finite-dimensional real vector spaces. What is the dual map α\alpha^{*} ? Explain why the rank of α\alpha^{*} is equal to the rank of α\alpha. Prove that the kernel of α\alpha^{*} is the annihilator of the image of α\alpha, and also that the image of α\alpha^{*} is the annihilator of the kernel of α\alpha.

[Results about the matrices representing a map and its dual may be used without proof, provided they are stated clearly.]

Now let VV be the vector space of all real polynomials, and define elements L0,L1,L_{0}, L_{1}, \ldots of VV^{*} by setting Li(p)L_{i}(p) to be the coefficient of XiX^{i} in pp (for each pVp \in V ). Do the LiL_{i} form a basis for VV^{*} ?