Fig. 1

Bibliometric map based on the network of co-authorship relations among the top 21 countries (with > 2 papers). Seven different colored clusters represent 94 total link strengths: cluster 1 (France, Germany, Switzerland, Italy, and Austria), cluster 2 (Canada and Belgium), cluster 3 (India and Australia), cluster 4 (South Korea, Taiwan, and Greece), cluster 5 (USA, Japan, Israel, and Norway), cluster 6 (China and Hong Kong), and cluster 7 (Sweden)