 Beginning in 1662 he traveled to the Netherlands, to study mathematics with Christiaan Huygens.In 1672 he published his first book, "Euclides Danicus", simultaneously in Copenhagen and Amsterdam, in Danish and Dutch respectively.This book, proving the Mohr–Mascheroni theorem 125 years earlier than Lorenzo Mascheroni, would languish in obscurity until its rediscovery in 1928.