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regular smooth curve - Penn Math

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Problem. To get a Frenet frame along a <strong>curve</strong> in R 3 , one<br />

needs to assume that the curvature �(s) never vanishes.<br />

Without this hypothesis, one can still prove that<br />

vol �-tube = (length of <strong>curve</strong>) (area of �-disk) .<br />

(a) Let T(s) , A(s) , B(s) be an ON frame along our<br />

<strong>curve</strong> x(s) . Show that the Frenet eqns are replaced by<br />

T'(s) = �(s) A(s) + �(s) B(s)<br />

A'(s) = � �(s) T(s) + �(s) B(s)<br />

B'(s) = � �(s) T(s) � �(s) A(s) .<br />

48

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