Claim: Haffner og Haagaas was a Norwegian series of textbooks in mathematics published in numerous editions between 1925 and 1979 and the series was originally edited by Theodor Haagaas (15 June 1873, Tistedalen – 25 December 1961, Oslo)  and Einar Haffner, for whom it was named.
Evidence: Haffner og Haagaas ("Haffner and Haagaas") was a Norwegian series of textbooks in mathematics published in numerous editions between 1925 and 1979, which were the most widely used textbooks in its field in Norwegian secondary schools for half a century. The series was originally edited by Theodor Haagaas and Einar Haffner, for whom it was named. Theodor Christian Petersen Haagaas (15 June 1873, Tistedalen – 25 December 1961, Oslo) was a Norwegian mathematician and private school owner.