Gustaf Enestr%C3%B6m
Gustaf Hjalmar Enestrm was a Swedish mathematician, statistician and historian of mathematics known for introducing the Enestrm index, which is used to identify Eulers writings. Most historical scholars refer to the works of Euler by their Enestrm index. With Soichi Kakeya, he is known for the EnestrmKakeya theorem which determines an annulus containing the roots of a real polynomial.
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Source: Wikipedia