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Cognomen Kogen (Kogen , Коген , Китамура ) , worldwide, is a common last name. The cognomen Kogen is characteristic of Japan, where it is quite a common surname, Austria, where it is a rare surname, and Russia. Not in comparison with a country's population, it is the most numerous in Japan, the United States, and Russia. Likewise, Kogen is the forename as well as the last name.