[WIP] Add more citations for historical completness (#609)

* Add Rotman's book to biblio

* Remove white spaces and add two per. entropy references

* Add reference to original persistence entropy paper in docstring

Co-authored-by: Umberto Lupo <[email protected]>

doc/theory/bibliography.bib

@@ -1,3 +1,28 @@
+@article{atienza2020entropy,
+	title = {On the stability of persistent entropy and new summary functions for topological data analysis},
+	journal = {Pattern Recognition},
+	volume = {107},
+	pages = {107509},
+	year = {2020},
+	issn = {0031-3203},
+	doi = {https://doi.org/10.1016/j.patcog.2020.107509},
+	url = {https://www.sciencedirect.com/science/article/pii/S0031320320303125},
+	author = {Nieves Atienza and Rocio Gonzalez-Díaz and Manuel Soriano-Trigueros}
+}
+
+@article{chintakunta2015entropy,
+	title = {An entropy-based persistence barcode},
+	journal = {Pattern Recognition},
+	volume = {48},
+	number = {2},
+	pages = {391-401},
+	year = {2015},
+	issn = {0031-3203},
+	doi = {https://doi.org/10.1016/j.patcog.2014.06.023},
+	url = {https://www.sciencedirect.com/science/article/pii/S0031320314002453},
+	author = {Harish Chintakunta and Thanos Gentimis and Rocio Gonzalez-Diaz and Maria-Jose Jimenez and Hamid Krim}
+}
+
 @article{morse1927rank,
 	AUTHOR = {Morse, Marston},
 	TITLE = {Rank and span in functional topology},
@@ -133,6 +158,13 @@ @incollection {edelsbrunner2002simplification
 	URL = {https://doi.org/10.1007/s00454-002-2885-2},
 }
 
+@book{rotman2008introduction,
+	title={An introduction to homological algebra},
+	author={Rotman, Joseph J},
+	year={2008},
+	publisher={Springer Science \& Business Media}
+}
+
 @article{zomorodian2005computing,
 	title={Computing persistent homology},
 	author={Zomorodian, Afra and Carlsson, Gunnar},
@@ -274,6 +306,4 @@ @book{stein2011functional
 	volume={4},
 	year={2011},
 	publisher={Princeton University Press}
-}
-
-
+}