1. On Finsler spaces of Douglas type-a generalization of the notion of Berwald space;Bácsó;Publ. Math. (Debr.),1997
2. On Douglas metrics;Chen;Publ. Math. (Debr.),2007
3. Riemann–Finsler Geometry;Chern,2005
4. The general geometry of paths;Douglas;Ann. Math.,1927
5. Partial Differential Equations;Evans,1998