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in mathematics, diophantine geometry is the study of points of algebraic varieties with coordinates in the integers, rational numbers, and their generalizations. these generalizations typically are fields that are not algebraically closed, such as number fields, finite fields, function fields, and p-adic fields (but not the real numbers which are used in real algebraic geometry). it is a sub-branch of arithmetic geometry and is one approach to the theory of diophantine equations, formulating questions about such equations in terms of algebraic geometry.
a single equation defines a hypersurface, and simultaneous diophantine equations give rise to a general algebraic variety v over k; the typical question is about the nature of the set v(k) of points on v with co-ordinates in k, and by means of height functions quantitative questions about the "size" of these solutions may be posed, as well as the qualitative issues of whether any points exist, and if so whether there are an infinite number. given the geometric approach, the consideration of homogeneous equations and homogeneous co-ordinates is fundamental, for the same reasons that projective geometry is the dominant approach in algebraic geometry. rational number solutions therefore are the primary consideration; but integral solutions (i.e. lattice points) can be treated in the same way as an affine variety may be considered inside a projective variety that has extra points at infinity.
the general approach of diophantine geometry is illustrated by faltings's theorem (a conjecture of l. j. mordell) stating that an algebraic curve c of genus g > 1 over the rational numbers has only finitely many rational points. the first result of this kind may have been the theorem of hilbert and hurwitz dealing with the case g = 0. the theory consists both of theorems and many conjectures and open questions.