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 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.