Add few references

matlab/ref.bib

@@ -197,3 +197,34 @@
   url             = {https://doi.org/10.1109/lsens.2019.2933065},
   tags            = {complementary filters},
 }
+
+@article{bendat57_optim_filter_indep_measur_two,
+  author = {J. Bendat},
+  title = {Optimum Filters for Independent Measurements of Two Related Perturbed Messages},
+  journal = {IRE Transactions on Circuit Theory},
+  year = {1957},
+  doi = {10.1109/tct.1957.1086345},
+  url = {https://doi.org/10.1109/tct.1957.1086345},
+  page = {--},
+}
+
+@article{plummer06_optim_compl_filter_their_applic_motion_measur,
+  author = {A. R. Plummer},
+  title = {Optimal Complementary Filters and Their Application in Motion Measurement},
+  journal = {Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering},
+  volume = {220},
+  number = {6},
+  pages = {489-507},
+  year = {2006},
+  doi = {10.1243/09596518JSCE229},
+  url = {https://doi.org/10.1243/09596518JSCE229},
+}
+
+@book{robert12_introd_random_signal_applied_kalman,
+  author = {Robert Grover Brown, Patrick Y. C. Hwang},
+  title = {Introduction to Random Signals and Applied Kalman Filtering with Matlab Exercises},
+  year = {2012},
+  publisher = {Wiley},
+  edition = {4},
+  isbn = {0470609699,9780470609699},
+}