At a point of a twice continuously differentiable regular curve with positive curvature , its osculating circle has radius and center , where is the principal normal vector. It matches the point, tangent line and curvature. For a space curve, it lies in the plane spanned by the unit tangent vector and principal normal vector. The parabola example under curvature of a plane curve gives center and radius at the origin.
An osculating circle is a circle that best approximates a curve at a given point. It is defined as the circle that has the same tangent and curvature as the curve at that point. In other words, the osculating circle touches the curve at that point and shares the same slope and curvature in a local neighborhood around that point.
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