Andrew Thangaraj
Aug-Nov 2020
\(V,W,U\): finite-dimensional inner product spaces over \(F=\mathbb{R}\) or \(\mathbb{C}\)
\((S+T)^* = S^* + T^*\), where \(S,T:V\to W\) are linear maps.
\((\lambda T)^* = \bar{\lambda}T^*\), where \(\lambda\in F\).
\((T^*)^*=T\).
\(I^*=I\), where \(I\) is the identity operator.
\((ST)^* = T^* S^*\), where \(S:W\to U\) and \(T:V\to W\) are linear maps.
Sample Proofs
\(\langle v,(S+T)^* w\rangle=\langle (S+T)v,w\rangle=\langle Sv,w\rangle+\langle Tv,w\rangle=\langle v,S^* w\rangle+\langle v,T^* w\rangle=\langle v,(S^* +T^*)w\rangle\)
\(\langle v,(\lambda T)^* w\rangle=\langle \lambda T v, w\rangle=\lambda\langle T v, w\rangle=\lambda\langle v, T^* w\rangle=\langle v, \bar{\lambda} T^* w\rangle\)
\(T:V\to W\), a linear map and \(T^*:W\to V\), adjoint of \(T\)
null \(T^* =\) \((\)range \(T)^{\perp}\)
range \(T^* =\) \((\)null \(T)^{\perp}\)
null \(T =\) \((\)range \(T^*)^{\perp}\)
range \(T =\) \((\)null \(T^*)^{\perp}\)
dim range \(T=\) dim range \(T^*\)
Proof
\(w\in\) null \(T^*\)
iff \(T^* w=0\), iff \(\langle v,T^* w\rangle=0\) for all \(v\in V\), iff \(\langle Tv,w\rangle=0\) for all \(v\in V\)
iff \(w\in\) \((\)range \(T)^{\perp}\)
(4) is complement of (1), (3) is (1) with \(T\) set as \(T^*\), (2) is complement of (3)
For (5), use fundamental theorem of linear maps on (3)
\(V,W\): finite-dimensional inner product spaces over \(F=\mathbb{R}\) or \(\mathbb{C}\)
\(T:V\to W\), a linear map
Orthonormal basis for \(V\): \(B_V=\{e_1,\ldots,e_n\}\)
Orthonormal basis for \(W\): \(B_W=\{f_1,\ldots,f_m\}\)
Matrix of \(T\) w.r.t. \(B_V\) and \(B_W\) is \(M(T,B_V,B_W)\)
\[M(T^*,B_W,B_V)=\text{conjugate-transpose}(M(T,B_V,B_W))\]
Conjugate-transpose of a matrix: transpose and conjugate each element
Proof
\((i,j)\)-th entry of \(M(T,B_V,B_W)\): \(\langle Te_j,f_i\rangle=\langle e_j,T^* f_i\rangle=\overline{\langle T^* f_i,e_j\rangle}\)
\((j,i)\)-th entry of \(M(T^* ,B_W,B_V)\): \(\langle T^* f_i,e_j\rangle\)
\(A\): \(m\times n\) matrix over \(\mathbb{R}\)
Represents \(T:\mathbb{R}^n\to \mathbb{R}^m\) w.r.t. standard basis
\(T^*\): represented by the conjugate-transpose\((A)\) or \(\overline{A^T}\)
Since entries of \(A\) are real: \(\overline{A^T}=A^T\)
range \(T^* =\) colspace \(A^T =\) rowspace \(A =\) \((\)null \(A)^{\perp}\)
null \(T^* =\) null \(A^T =\) left-null \(A =\) \((\)colspace \(A)^{\perp}\)
range \(T =\) colspace \(A =\) rowspace \(A^T =\) \((\)left-null \(A)^{\perp}\)
null \(T =\) null \(A =\) left-null \(A^T =\) \((\)rowspace \(A)^{\perp}\)
\(T:V\to W\) and \(S:W\to U\) are linear maps
\(ST:V\to U\) is a linear map
Suppose \(N_W=\) range \(T\cap\) null \(S\) (a subspace of \(W\))
dim null \(ST=\) dim null \(T+\) dim \(N_W\)
Proof
\(v\in\) null \(ST\) iff \(S(Tv)=0\) iff \(Tv\in\) null \(S\) iff \(Tv\in N_W\)
null \(ST=\{v:Tv\in N_W\}\)
Basis for null \(T\): \(\{v_1,\ldots,v_k\}\)
Basis for \(N_W\): \(\{w_1,\ldots,w_r\}\), \(r=\) dim \(N_W\)
Let \(v_{k+i}\in V\) be s.t. \(Tv_{k+i}=w_i\), \(i=1,\ldots,r\)
Basis for null \(ST\): \(\{v_1,\ldots,v_k,v_{k+1},\ldots,v_{k+r}\}\)
\(T:V\to W\) and \(S:W\to U\) are linear maps
\(ST:V\to U\) is a linear map
if range \(T\) and null \(S\) intersect only at 0, null \(ST=\) null \(T\)
\(S\) retains all “details” of \(T\)
By fundamental theorem of linear maps, dim range \(ST=\) dim range \(T\)
\(T:V\to W\) and \(T^*: W\to V\)
\(T^*T: V\to V\)
null \(T^* =\) \((\)range \(T)^{\perp}\)
So, null \(T^*\) and range \(T\) intersect only at \(0\)
Therefore, null \(T^* T=\) null \(T\) and dim range \(T=\) dim range \(T^* T=\) dim range \(T^*\)
\(TT^*: W\to W\)
null \(T =\) \((\)range \(T^*)^{\perp}\)
So, null \(T\) and range \(T^*\) intersect only at \(0\)
Therefore, null \(TT^* =\) null \(T^*\) and dim range \(T=\) dim range \(TT^*=\) dim range \(T^*\)