Difference Equations and Functional Equations. 



6000 General. 



6010 Recurring" series. 



6020 Solution of equations of finite differences. 



6030 Solution of functional equations. {See also 4460.) 



GEOMETRY. 



Foundations. 



6400 General. 



6410 Principles of geometry ; non-Euclidean geometries ; 



hyperspace. 

 6420 Topology of space and hyperspace. 

 6430 Methods of analytical geometry. {See also 0840.) 



Elementary Geometry. 



6800 General. 



6810 Planimetry ; straight lines, and circles. 



6820 Stereometry ; straight lines, planes, and spheres. 



6830 Trigonometry. 



6840 Descriptive geometry ; perspective. 



Geometry of Conies and Quadrics. 



7200 General. 



7210 Metrical properties of conies. 



7220 Projective properties of conies. 



7230 Systems of conies. {See also 8070.) 



7240 Metrical properties of qnadric surfaces. 



7250 Projective properties of quadric surfaces. 



7260 Systems of quadric surfaces. {See also 8070.) 



Algebraic Cur\res and Surfaces of degree higher than 

 the second. 



7600 General. 



7610 Metrical properties of algebraic plane curves of degree 



higher than tlie second. 

 7620 Projective properties of algebraic plane curves of 



degree higher than the second. {See also 8030.) 

 7630 Special plane algebraic curves. 

 7640 Algebraic surfaces of degree higher than the second. 



{See also 8040.) 

 7650 Special algebraic surfaces. 

 7660 Skew algebraic curves. {See also 8030.) 



Transformations and General Methods for Algebraic 

 Configurations. 



8000 General. 



8010 Collineatiou ; duality. 



8020 Other algebraic transformations. 



