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Weyl's tube formula

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Weyl's tube formula gives the volume of an object defined as the set of all points within a small distance of a manifold.

Let Σ {\displaystyle \Sigma } be an oriented, closed, two-dimensional surface, and let N ε ( Σ ) {\displaystyle N_{\varepsilon }(\Sigma )} denote the set of all points within a distance ε {\displaystyle \varepsilon } of the surface Σ {\displaystyle \Sigma } . Then, for ε {\displaystyle \varepsilon } sufficiently small, the volume of N ε ( Σ ) {\displaystyle N_{\varepsilon }(\Sigma )} is

V = 2 A ( Σ ) ε + 4 π 3 χ ( Σ ) ε 3 , {\displaystyle V=2A(\Sigma )\varepsilon +{\frac {4\pi }{3}}\chi (\Sigma )\varepsilon ^{3},}

where A ( Σ ) {\displaystyle A(\Sigma )} is the area of the surface and χ ( Σ ) {\displaystyle \chi (\Sigma )} is its Euler characteristic. This expression can be generalized to the case where Σ {\displaystyle \Sigma } is a q {\displaystyle q} -dimensional submanifold of n {\displaystyle n} -dimensional Euclidean space R n {\displaystyle \mathbb {R} ^{n}} .

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