diff --git a/README.md b/README.md
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@@ -256,7 +256,7 @@ Our journey through the quantum realm is inspired by the following:
    6. Aerts, D., Beltran, L. *Quantum Structure in Cognition: Human Language as a Boson Gas of Entangled Words*. Found Sci 25, 755–802 (2020). [https://doi.org/10.1007/s10699-019-09633-4](https://doi.org/10.1007/s10699-019-09633-4)
    7. Beiser, A. (2003). *Concepts of Modern Physics*. 6th ed. McGraw Hill. ISBN: [0072448482](https://www.worldcat.org/isbn/0072448482) ([9780072448481](https://www.worldcat.org/isbn/9780072448481))
    8. Celeghini, E., Gadella, M., & del Olmo, M. A. (2021). *Hermite functions and Fourier series*. Symmetry, 13(5), Article 853. [https://doi.org/10.3390/sym13050853](https://doi.org/10.3390/sym13050853)
-   9. Schleich, W. P. (2001). *Quantum optics in phase space*. Wiley-VCH. ISBN:[352729435X](https://www.worldcat.org/isbn/352729435X) ([9783527294350](https://www.worldcat.org/isbn/9783527294350))
+   9. Schleich, W. P. (2001). *Quantum optics in phase space*. Wiley-VCH. ISBN: [352729435X](https://www.worldcat.org/isbn/352729435X) ([9783527294350](https://www.worldcat.org/isbn/9783527294350))
    10. Leonhardt, U. (2010). Essential Quantum Optics: From Quantum Measurements to Black Holes. Cambridge: Cambridge University Press. ISBN: [0521869781](https://www.worldcat.org/isbn/0521869781) ([9780521869782](https://www.worldcat.org/isbn/9780521869782))
    11. Pérez-Jordá, J. M. (2017). *On the recursive solution of the quantum harmonic oscillator*. European Journal of Physics, 39(1), 015402. [https://doi.org/10.1088/1361-6404/aa9584](https://doi.org/10.1088/1361-6404/aa9584)
    12. Olver, F. W. J., & Maximon, L. C. (2010). *NIST Handbook of Mathematical Functions*. Cambridge University Press. ISBN: [0521192250](https://www.worldcat.org/isbn/0521192250) ([9780521192255](https://www.worldcat.org/isbn/9780521192255))