Paper 2, Section II, F

Linear Algebra | Part IB, 2016

Let Mn,nM_{n, n} denote the vector space over a field F=RF=\mathbb{R} or C\mathbb{C} of n×nn \times n matrices with entries in FF. Given BMn,nB \in M_{n, n}, consider the two linear transformations RB,LB:Mn,nR_{B}, L_{B}: M_{n, n} \rightarrow Mn,nM_{n, n} defined by

LB(A)=BA,RB(A)=ABL_{B}(A)=B A, \quad R_{B}(A)=A B

(a) Show that detLB=(detB)n\operatorname{det} L_{B}=(\operatorname{det} B)^{n}.

[For parts (b) and (c), you may assume the analogous result detRB=(detB)n\operatorname{det} R_{B}=(\operatorname{det} B)^{n} without proof.]

(b) Now let F=CF=\mathbb{C}. For BMn,nB \in M_{n, n}, write BB^{*} for the conjugate transpose of BB, i.e., B:=BˉTB^{*}:=\bar{B}^{T}. For BMn,nB \in M_{n, n}, define the linear transformation MB:Mn,nMn,nM_{B}: M_{n, n} \rightarrow M_{n, n} by

MB(A)=BABM_{B}(A)=B A B^{*}

Show that detMB=detB2n\operatorname{det} M_{B}=|\operatorname{det} B|^{2 n}.

(c) Again let F=CF=\mathbb{C}. Let WMn,nW \subseteq M_{n, n} be the set of Hermitian matrices. [Note that WW is not a vector space over C\mathbb{C} but only over R.]\mathbb{R} .] For BMn,nB \in M_{n, n} and AWA \in W, define TB(A)=BABT_{B}(A)=B A B^{*}. Show that TBT_{B} is an R\mathbb{R}-linear operator on WW, and show that as such,

detTB=detB2n\operatorname{det} T_{B}=|\operatorname{det} B|^{2 n}

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