The sequence of matrices C_y for each year y from 1991 to 2020.
Let P be the set of articles with at least two authors from different European countries then
C_y[c,d] = cardinality (number of members) of the set of articles published in the year y with an author from the country c and an author from the country d;
C_y[c,c] = cardinality (number of members) of the set of articles from P published in the year y with an author from the country c.
