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| | PlanetMath: geometry (Site not responding. Last check: 2007-11-07) |
 | | Early differential geometers studied such properties of curves and surfaces such as: computing their lengths and areas, finding tangents, constructing evolute, involute, and pedal curves, studying curvature and osculating circles, and finding envelopes and orthogonal curves to a given family of curves. |
 | | In the case of a surface, the situation is a little more complicated -- to describe the direction of the normal vector, one needs two angles instead of one and one can choose to compute their directional derivatives along any direction tangent to the surface. |
 | | Hence, to define the notion of metric field, we will consider the assignment of a symmetric, positive definite matrix of functions to every coordinate system in such a way that the matrices assigned to two coordinate systems are related according to the transformation law for inner products under a change of basis. |
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