## 3 Bibliometric Analysis
### Preliminary Analysis of Publications  
Before starting with the analysis of co-citations, it is necessary to know the main features of the articles to be analysed. To achieve this, a preliminary analysis of the set of articles to be studied is performed. This preliminary analysis is carried out to check whether this topic is really interesting; thus, the result of this analysis will be the answer to the question \"Is the study of air traffic complexity really relevant?\" To carry out the study of the interest of complexity, the first subjects to study are the number of articles published and the number of citations per year. Figure 2 shows this first analysis. The publications are shown in grey, together with their trend from 2001 to 2022. The number of citations is also shown in blue, subject to the secondary axis. Figure 2 shows how the number of publications and citations has grown overall since 2001. It has gone from publishing fewer than 2 articles per year until 2004 to publishing more than 10 articles in 2021 and 2022 related to this topic based only on the WoS repository. Moreover, the trend is to increase this number in the following years. Specifically, this trend has been stable since 2001, except in the period 2009-2014, where there is a sudden increase in publications. From this moment, the slope of the trend of publications is lower, although increasing until 2022. As for the number of citations, this has also increased in general and much more in absolute value. Specifically, more than 150 citations are reached in 2021 and 2022. It can be deduced from the citation curve that interest in the topic is high and has increased in recent years. This trend lags slightly behind that of the number of publications, which is logical as these papers cannot be cited until they are published and disseminated among the scientific community. Despite being a very specific topic within the aeronautics industry, and currently publishing around 10 papers per year, the number of citations to these is much higher. Specifically, the number of citations is more than 10 times per publication. This is a very large number and it is estimated that these papers are very well received in the general industry and that they are cited in articles dealing with other topics. Figure 2 shows that the complexity of aerospace is indeed a topic that is attracting a great deal of interest, especially in recent years. In addition, the graph of the number of publications related to the WoS categories is presented. The publications studied are divided into 45 different categories. However, Figure 3 shows only the 10 categories with the most publications. Figure 2: Publications and citations related to air traffic complexity from 2001 to 2022. Below is a list of acronyms for all the categories used in Figure 3 for a better understanding of the figure. * Trans. (Transport);
* Eng. (Engineering);
* Aut. (Automation);
* Sys. (Systems);
* AI (Artificial Intelligence);
* O.R.M. (Operation Research Management). It can be seen from Figure 3 that the most common category is \"Engineering Aerospace\". This is logical, as the problem of complexity in aerospace is an aerospace problem. A total of 42 publications (30% of the total) are included in this topic. The topic \"Engineering Electrical Electronic\" gathers 36 publications (25%); this is because a large part of the solutions proposed in the papers to solve this problem are related to this field. The third most common topic is \"Transportation Science Technology\", with 28 publications (19%). This is also logical. On the other hand, it is interesting to note that \"Computer Science Artificial Intelligence\" only has eight publications in the whole period studied. Artificial Intelligence (AI)-based solutions are gaining importance in recent years, and the complexity of aerospace is benefiting from the advances that AI brings [16; 41].