This library has more than 150,000 books in the subjects it covers. The school also has specialised libraries in archeology, cognitive sciences, mathematics and computer science, theoretical physics. A recently unified natural sciences library was opened in 2013, aiming to bring together in a central place on rue d'Ulm the libraries of physics, chemistry, biology and geoscience. The school also features two specialised centres for documentation, the "Bibliothèque des Archives Husserl", and the "Centre d'Archives de Philosophie, d'Histoire et d'Edition des Sciences. Two other "écoles normales supérieures" were established in the 20th century: the École Normale Supérieure de Lyon (sciences and humanities); and the École normale supérieure Paris-Saclay (pure and applied sciences, sociology, economics and management, English language). More recently, the fourth "école normale supérieure" was created on January 2014 under the name of École Normale Supérieure de Rennes (pure and applied sciences, economics and management, law school, sport) in Brittany. All four together form the informal ENS-group. The École normale supérieure is also an institution of PSL Research University, a union of several higher education institutions, all located in Paris, which aims at achieving cooperation and developing synergies between its member institutions to promote French research abroad. In addition to this, the École normale supérieure cooperates in Atomium Culture, the first permanent platform for European excellence that brings together some of Europe's leading universities, newspapers and businesses. The school is a member of the Conference of University Presidents and of the Conference of Grandes Ecoles. Its educational project being based on research, ENS seeks to train its students to become researchers. The main objective of the education given is getting a doctorate, and more than 85% of "normaliens" achieve this. The students are free to choose their own course of study but must at least attain a master's degree in research. Since, traditionally, the institution does not have the powers to grand university degrees, this entails that students have to follow courses in other universities in Paris. To this end, ENS cultivates a large number of partnerships and conventions with other higher education institutions to create master's degrees which are co-presided by two institutions. ENS works closely with the Ecole des Hautes Etudes en Sciences Sociales (EHESS), the Paris-Sorbonne University, the Panthéon-Sorbonne University, HEC Paris and ESSEC Business School in particular to deliver joint diplomas to a certain number of students who have followed courses shared between the two institutions.