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Leonhard Euler (1707-1783), Swiss mathematician. Euler developed the theory of differential equations and the calculus of variations, and did important work in astronomy and optics. He worked in St Petersburg, Russia, and in Berlin, Germany, publishing over 800 papers. He became blind in the last years of his life, but continued his work by calculating in his head. Engraving from Les Savants Modernes (1899).
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