# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 4 Conclusions
These small communities are considered emerging communities where the greatest progress has been made, as most have appeared and fully developed in the last decade. An example of this is the community where the biological effects of ELF fields on crops are studied, where most of the papers have been published between 2008 [103] and 2014 [123]. These papers consider the exposure of different crops to ELF fields, which may be a discriminating factor for their growth. A similar fact occurs in another smaller community studied, the one related to the UELF spectrum. This community is formed by a small number of works developed since 2017 [124], so it is a very recent and little-researched topic. These are some of the reasons why these communities raise the possibility of developing many future works and research. The subject studied has great potential and can be applied to many fields, giving rise to a great deal of research and international publications of great interest. Despite this, there are communities that have disappeared, as is the case of the geomagnetic pulsations community, in which there have been no publications since 1983. The most-repeated keywords globally correspond to the word used in the search: \"extremely low frequency\". There are also generic words that appear among the most-repeated in most communities individually: \"electromagnetic fields\", \"magnetic fields\", and \"extremely low frequency electromagnetic fields\". On a global level, specific keywords from each community also appear, such as \"Schumann resonance\", typical of the lightning ELF phenomena community or \"cancer\", a word located among the 10 most-repeated in the ELF in health community. In total, there are 28,875 authors who investigate, 27 by subject, with the United States (6023 researchers) being the country that contributes the most researchers to this area, followed by China (3072 researchers) and Japan (2088 researchers). If the 20 authors with the highest H-index are analyzed according to Scopus in the search carried out, it is verified that it does not correspond directly with the distribution of authors by countries, with the exception of the first two that are from the United States, the country with the highest contribution of authors. The Chinese researchers are not among these top 20, the first being in position 21, while the first researcher from the next country for contributions, Japan, is in position 16. In addition, authors who publish in the most-represented community (ELF in health) show the great interest generated by the effects of ELF fields on health (58.50%). Of the 7849 centers that investigate ELF waves, the main collaborations are established between European centers, followed in isolation from Japanese, Chinese, and United States centers, this division coinciding with that of the countries that publish the most on this subject. The temporal evolution of the documents published on Scopus shows a continuous growth from 1956 to 2018, at which point there is a small decrease as shown in 2019. This trend is shown both in the documents derived from the initial search, and in the documents studied after the purification and analysis process, observing in this line less fluctuations in time. The main form of publication of the documents found is article format (77.86%), followed by conference paper (13.14%), review (3.76%), and book chapter (1.78%). The rest of the formats constitute 2.23% of the total published documents. As for the language, English (92.00%) is the most used to disseminate publications due to the standards of edition of the journals in order to have maximum dissemination regardless of the country of origin or the headquarters of the publisher. The second language used is Chinese (1.97%) since it produces a significant number of scientific publications with international repercussions in their own language. The rest of the languages have a representation of less than \(<\)1.80%.