feat(latex): add new book reference for discrete-time signal processing

and clarify mathematical definition with citation

latex/bibliography.bib

@@ -827,6 +827,20 @@
   file = {C\:\\Users\\damar\\Zotero\\storage\\6XITVIKY\\toh2020.pdf.pdf;C\:\\Users\\damar\\Zotero\\storage\\9L9KXB7V\\Toh and Park - 2020 - Review of Vibration-Based Structural Health Monito.pdf}
 }
 
+@book{oppenheim2010,
+  title = {Discrete-Time Signal Processing},
+  editor = {Oppenheim, Alan V. and Schafer, Ronald W.},
+  date = {2010},
+  series = {Prentice {{Hall}} Signal Processing Series},
+  edition = {3. ed},
+  publisher = {Pearson},
+  location = {Upper Saddle River Munich},
+  isbn = {978-0-13-198842-2},
+  langid = {english},
+  pagetotal = {1108}
+}
+
+
 @article{vos2022,
   title = {Vibration-Based Anomaly Detection Using {{LSTM}}/{{SVM}} Approaches},
   author = {Vos, Kilian and Peng, Zhongxiao and Jenkins, Christopher and Shahriar, Md Rifat and Borghesani, Pietro and Wang, Wenyi},