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2. Technical features of tillage tools;Clyde;Bull. 465, Pt 2, Agric. Exp. Stn, Penn. St. Coll.,1944
3. Messung der Kraftkomponenten an einem Pflugkörper;von Getzlaff;Grundl. Landtech.,1951
4. Vector and tensor analysis;Lass,1950
5. Principles of mechanics;Synge,1949