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Euler's career in mathematics began in Russia, where he joined the St. Petersburg Academy of Sciences in 1727 upon the recommendation of his friend Daniel Bernoulli. His tenure there allowed him to engage in collaborative work with esteemed mathematicians such as Jakob Hermann and Christian Goldbach. Euler tackled several problems, one notably being the "Seven Bridges of Königsberg," which earned him repute as a pioneer in graph theory. Euler established the fundamental theorem that to traverse all bridges without repetition, the nodes must possess specific properties concerning their degrees, forever changing how mathematicians approach similar problems.