Adjoint of linear Operator and T-invariant subspace

In summary, the adjoint of a linear operator is the transpose or complex conjugate transpose of the matrix representing the linear transformation. It is defined as a unique linear operator with the property that the inner product of A*u and v is equal to the inner product of u and A*v. A T-invariant subspace is a subspace that remains unchanged under a linear transformation T. To determine if a subspace is T-invariant, the basis vectors can be checked to see if they are also mapped to vectors in the subspace. The adjoint of a linear operator is T-invariant if the subspace spanned by its eigenvectors is T-invariant. This relationship is important in applications such as quantum mechanics.
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Homework Statement



Let T be a linear operator on an inner product space V and W be a T-invariant subspace of V. If W is both T and T* invariant, prove that (T[itex]_{W}[/itex])* = (T*)[itex]_{W}[/itex]. Note that T[itex]_{W}[/itex] denotes the restriction of T to W

Homework Equations



[itex]\forall[/itex]x[itex]\in[/itex]W, T[itex]_{W}[/itex](x) = T(x)
T(W)[itex]\subseteq[/itex]W
T*(W)[itex]\subseteq[/itex]W

The Attempt at a Solution



I attempt to show that the image of any x [itex]\in[/itex]W under (T[itex]_{W}[/itex])* and (T*)[itex]_{W}[/itex] are the same.
<T[itex]_{W}[/itex]*(x), y> = < x, T[itex]_{W}[/itex](y)> = < x, T(y) > = < T*(x), y> = <(T*)[itex]_{W}[/itex](x), y >

Since this is true for all y, therefore, (T[itex]_{W}[/itex])* = (T*)[itex]_{W}[/itex].

Is the above proof correct? I feel that there is something wrong in one of the steps.
Thank You.
 
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  • #2




Your proof looks correct to me. You have correctly shown that the images of any x in W under (T_W)* and (T*)_W are the same, which means that the two operators are equal. Well done!
 

1. What is the adjoint of a linear operator?

The adjoint of a linear operator is a concept in linear algebra that is used to find the "opposite" of a given linear transformation. It is essentially the transpose of the matrix representing the linear operator, but can also be thought of as the complex conjugate transpose in the case of complex vector spaces.

2. How is the adjoint of a linear operator defined?

The adjoint of a linear operator A, denoted as A*, is defined as the unique linear operator on the same vector space with the property that for any two vectors u and v, the inner product of A*u and v is equal to the inner product of u and A*v.

3. What is a T-invariant subspace?

A T-invariant subspace is a subspace of a vector space that remains unchanged under a linear transformation T. This means that for any vector v in the subspace, the image of T(v) is also in the subspace.

4. How do you determine if a subspace is T-invariant?

To determine if a subspace is T-invariant, you can check if the linear transformation T maps every vector in the subspace to another vector in the subspace. This can be done by checking if the basis vectors of the subspace are also mapped to vectors in the subspace.

5. How are the adjoint of a linear operator and T-invariant subspace related?

The adjoint of a linear operator A is T-invariant if and only if the subspace spanned by the eigenvectors of A* is T-invariant. In other words, the eigenspaces of A* are T-invariant subspaces. This relationship is important in applications such as quantum mechanics, where the adjoint operator represents the Hermitian conjugate of a quantum observable.

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