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'fine tune dagger'
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DPotoyan authored Oct 14, 2024
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### Hermitian property of operators

- What would be an analog of complex conjugate for matrices?
- This leads us to defined **adjoint of an operator matrix/operator $A^{\dag}$** which is obtained by swapping indices and taking complex conjugate of all elements.
- This leads us to defined **adjoint of an operator matrix/operator $A^{\dagger}$** which is obtained by swapping indices and taking complex conjugate of all elements.
- With same analogy when matrix is equal to its adjoint its eigenvalues are real!
- Such matrices are called Hermitian or self-adjoint.

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:::

- To see that Differentiation operators are Hermitian requires a little more work. - A trick that helps see it is integration by parts where the constant term is zero because wavefunction decays to zero at boundaries (postulate 1, keeping probability finite)!
- To see that Differentiation operators are Hermitian requires a little more work.

- A trick that helps see it is integration by parts where the constant term is zero because wavefunction decays to zero at boundaries (postulate 1, keeping probability finite)!

$$\int \psi_1 d\psi_2 =- \int \psi_2d\psi_1 + \psi_1\psi_2\Big|_{x_{min}}^{x_{max}} =- \int \psi_2d\psi_1$$

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\int \psi \left(\hat{A} \psi\right)^* \, d\tau = a^*
$$

- Since the operator is Hermitian, this leads to the conclusion:
- Since the operator is Hermitian, this leads to equality ensuring real nature of eigenvalues.

$$
a = a^*
$$

- Thus, the eigenvalue $a$ must be real.




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\textnormal{RHS: } \int \psi_k \left(\hat{A} \psi_j \right)^* \, d\tau = \int \psi_k \left(a_j \psi_j \right)^* \, d\tau = a_j \int \psi_j^* \psi_k \, d\tau
$$

Since the operator is Hermitian, we require that LHS = RHS. This results in:
- Since the operator is Hermitian, we require that LHS = RHS. This results in:

$$
\left(a_k - a_j \right) \int \psi_j^* \psi_k \, d\tau = 0
$$

If $a_j \neq a_k$, then we have:
- If $a_j \neq a_k$, then we have:

$$
\int \psi_j^* \psi_k \, d\tau = 0
$$

This shows that $\psi_j$ and $\psi_k$ are orthogonal.
- This shows that $\psi_j$ and $\psi_k$ are orthogonal.

- **Note**: If $a_j = a_k$, meaning the eigenvalues are degenerate, this result does not hold. Degeneracy refers to eigenstates having the same eigenvalue, and in that case, orthogonality may not apply without further specification.
- **Note**: If $a_j = a_k$, meaning the eigenvalues are degenerate, this result does not hold.


### Problems
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**A Matrix**

To check if a matrix is Hermitian, it must satisfy the condition $A = A^\dagger \), where $A^\dagger$ is the conjugate transpose of $A$. Since this matrix has real entries, the conjugate transpose is just the transpose.
To check if a matrix is Hermitian, it must satisfy the condition $A = A^\dagger$, where $A^\dagger$ is the conjugate transpose of $A$. Since this matrix has real entries, the conjugate transpose is just the transpose.

The transpose of $A$ is:

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