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from the abstract: "assessing the influence of spatial proximity between scientists is crucial to promote efficient collaboration strategies and, ultimately, to improve the quality of science. Here we present a systematic analysis of citation and collaboration streams between cities and countries. By assigning papers to the geographic locations of their authors' affiliations, we construct weighted networks of citations and collaborations. The citation flows as well as the collaboration strengths between cities decrease with the distance between them and follow gravity laws with exponents close to 1. Moreover, for a given number of authors, the diversity of affiliations increases the number of citations, especially when many countries are represented. In addition, the total research impact of a country grows linearly with the amount of national funding for research & development. However, the average impact reveals a peculiar threshold effect: the scientific output of a country may reach an impact larger than the world average only if the country invests more than 120,000 US $ per researcher annually. Our results reveal the overall structure of scientific research by showing the correlation between collaboration, citation, geography and funding, and could provide valuable inputs in shaping the future science policies."