Danish mathematician. Initially taught mathematics by his parents, Mohr later studied in Holland, France, and England. He fought in the Dutch-French wars, and was briefly a prisoner of war. Mohr was little known in mathematics until the rediscovery of his lost book Euclides danicus (1672) in a bookstore in 1928. The book contains the theorem and its proof that all Euclidean constructions can be carried out with compasses alone, a result not found again until 125 years later.