improved periodic extension notebook to cover case where using both even and odd extensions is helpful

Author: pavelkomarov

notebooks/README.md

@@ -11,4 +11,4 @@
 | [DFT, DCT, DST Relationship](https://github.com/pavelkomarov/spectral-derivatives/blob/main/notebooks/dft_dct_dst_relationship.ipynb) | Explicit formulae and code examples to show how the coefficients of DCT and DST variants are related to DFT coefficients. A mathematical derivation of a couple of these relationships lives in [the math](https://pavelkomarov.com/spectral-derivatives/math.pdf), but it's useful to see others too and have a quick reference.|
 | [DFT Play](https://github.com/pavelkomarov/spectral-derivatives/blob/main/notebooks/dft_play.ipynb) | Due to aliasing, it's possible to find the DFT coefficients with many different combinations of wavenumbers, so long as they cover $\{0, ...N\}$ modulo $N$. Here lies some code to demonstrate this fact.|
 | [Fourier](https://github.com/pavelkomarov/spectral-derivatives/blob/main/notebooks/fourier.ipynb) | Usage examples of `fourier_deriv`, in one and multiple dimensions, with an emphasis of when this method will work and when it will break.|
-| [Fourier Periodic Extensions](https://github.com/pavelkomarov/spectral-derivatives/blob/main/notebooks/fourier_periodic_extensions.ipynb) | It's possible to take even and odd extensions of a function and produce something periodic or nearly periodic, which we can then spectrally differentiate with the Fourier basis. This notebook demonstrates how to do this.|
+| [Fourier Periodic Extensions](https://github.com/pavelkomarov/spectral-derivatives/blob/main/notebooks/fourier_periodic_extensions.ipynb) | It's sometimes possible to take even and odd extensions of a function and produce something periodic or nearly periodic, which can then be spectrally differentiated with the Fourier basis. This notebook demonstrates how to do this.|