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'test cards'
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DPotoyan authored Oct 14, 2024
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**For operators**

$${\hat{A}\psi_i(x,y,z) = A_n\psi_i(x,y,z)}$$
$${\hat{A}\psi_n = A_n\psi_n}$$

- This is an eigenvalue problem solution of which yields $n = 1,2,3,...$ number of eigenfunctions $\psi_i$ and the eigenvalues $E_i$. Depending on boundary conditions there could be finite or infinite number of solutions.
- This is an eigenvalue problem solution of which yields $n = 1,2,3,...$ number of eigenfunctions $\psi_n$ and the eigenvalues $E_i$. Depending on boundary conditions there could be finite or infinite number of solutions.

:::{note} **Example: find eigenvalues and eigenfunctions of momentum operator**
:::{admonition} **Example: find eigenvalues and eigenfunctions of momentum operator**
:class: note

What are the eigenfunctions and eigenvalues of an operator $\hat{A} = d/dx$
:::

:::{note} **Solution**
:::{{admonition} **Solution**
:class: dropdown

Finding eigenfunctions/eigenvalue of differnetial operators analytically involves solving differnetial equations
Expand All @@ -40,7 +40,7 @@ $${f_k = e^ce^{kx} = c'e^{kx}}$$

$$Av = \lambda v$$

:::{note} **Example: finding eigenvalues of a matrix**
:::{admonition} **Example: finding eigenvalues of a matrix**
:class: note

$$\begin{pmatrix}
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\end{pmatrix}$$
:::

::::{admonition}
:::{admonition}

```python
:::python
import numpy as np

# Define the matrix
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# Display the eigenvalues and eigenvectors
eigenvalues, eigenvectors
```
::::
:::

:::



### Eigenfunctions of Hermitian operators form complete basis set

The three crucial consequences of Hermitian property of operators

::::{grid}
:gutter: 2

### Eigenfunctions of Hermitian operators form complete basis set
:::{grid-item-card} Integral Notation

The three crucial consequences of Hermitian property of operators
$$\int \phi^* \hat{H}\psi dx = \int \psi (\hat{H}\phi)^*dx$$

:::

:::{grid-item-card} Dirac Notation

$$\langle \phi \mid \hat{H} \mid \psi \rangle = \langle \psi \mid \hat{H}\mid \phi \rangle^*$$

- Eigenvalues are real:
:::


::::



- **Eigenvalues are real**:

$$\hat{H} \mid \psi_n \rangle=E_n \mid \psi_n \rangle$$

$$E_n=E^*_n$$

- Eigenfunctions are orthogonal (and can also be normalized)
- **Eigenfunctions are orthogonal**

$$\langle \psi_n \mid \psi_m\rangle=\delta_{nm}$$

- Eigenfunctions form a complete basis set!
- **Eigenfunctions form a complete basis set!**

$$\mid f\rangle = \sum_i c_i \mid \psi_i \rangle$$

- The last two properties imply that eigenfunctions of Hermitian opeartors play the same role for functions as the unit vectors for vectors. That is a function can be expressed in terms of the eigenfunctions of an opearators which can act on the function.
- The last two properties imply that eigenfunctions of Hermitian opeartors play the same role for functions as the unit vectors for vectors. Thus a wavefunction can be expressed in terms of the eigenfunctions of an opearators which can act on the function.

### Wave function as a linear superoposition of eigenfunctions

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