## 3 The Results of the Bibliometric Analysis
### Social Structure Analysis
#### 3.2.3 Countries and International Cooperation Analysis  
In terms of geographical distribution, a total of 55 countries and regions have participated in publishing 402 articles and reviews, while 30.9% of these countries contributed only one paper. Figure 7 illustrates the geographical distribution of publications from 2000 to 2023. The result shows that China took the lead with the largest number of publications at 177, followed by the United States with 68, England with 34, South Korea with 29 and Poland with 25. When it comes to the total number of citations in Table 3, China (1655), the United States (1315), South Korea (702) and England (685) stood out, highlighting the substantial influence of these countries in this field. Despite a smaller quantity of publications, Bangladesh and (413) Sweden (61.5) received a higher average number of citations than most other countries, indicating the exceptional quality of research conducted in these two nations. Overall, as per the statistics, the developing countries indeed had a limited global impact in the field of the USV in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rank** & **Country** & **TC** & **AC** \\ \hline
1 & China & 1655 & 8.40 \\
2 & USA & 1315 & 19.39 \\
3 & South Korea & 702 & 35.10 \\
4 & England & 685 & 20.83 \\
5 & Bangladesh & 413 & 413.00 \\
6 & Poland & 332 & 17.47 \\
7 & Slovenia & 231 & 23.10 \\
8 & Australia & 223 & 27.88 \\
9 & Finland & 157 & 31.40 \\
10 & Norway & 146 & 12.17 \\ \hline \hline \end{tabular} Notes: TC = times cited; AC = average number of citations per publication.  
\end{table} 
Table 3: Total citation of the top ten countries. Figure 7: The distribution of publications arising from various countries. terms of publications and citations, as only two developing countries ranked within the top 15 productive countries. To find the internal connection in the distribution, the international cooperation was analyzed by mapping countries based on the co-authorship. The detailed setting for the parameters used for the figure is provided in Appendix A Table A2. It is well known that close research collaborations between countries drive technological and research advancements. Figure 8 illustrates the collaborative relationship among countries by examining co-authorship patterns. It is worth noting that the substantial number of publications on the new technology of USVs originated from China, as there was significant international cooperation between China and other countries, indicating that the research in this field actually requires a particular international cooperation. The network was constructed by using the full-counting method, with a minimum threshold of two documents per country, leading to the inclusion of 35 countries in the analysis. To normalize the data, association strength was applied. The color-coded overlay represents the year of publication, and the thickness of the links is proportional to the overall strength of the collaborations. The international collaboration network reveals the presence of four primary clusters. Notably, China emerged as the most active collaborator, with the strongest link (10) established with England. It can also be inferred that Chinese researchers were currently responsible for the majority of document productions, as indicated by the color-coded data, and their collaborative network extended prominently at the international level. Additionally, note that a strong circle featured by the European region is shown in the red cluster, where Italy dominated the cooperative links in the red cluster, primarily conducting collaboration with England, Germany and Greece, along with some publications for a single time with other countries. South Korea and Poland took the privilege to engage in more contact with other countries in their clusters, respectively. Figure 9 also presents information about the average publication year of each country, an indicator that tells us which country is more in tune with the era.