Third fundamental form: Difference between revisions
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{{unreferenced|date=October 2015}} |
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In [[differential geometry]], the '''third fundamental form''' is a surface metric denoted by <math>\ |
In [[differential geometry]], the '''third fundamental form''' is a surface metric denoted by <math>\mathrm{I\!I\!I}</math>. Unlike the [[second fundamental form]], it is independent of the [[surface normal]]. |
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==Definition== |
==Definition== |
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Let |
Let {{mvar|S}} be the [[shape operator]] and {{mvar|M}} be a [[smooth surface]]. Also, let {{math|'''u'''<sub>''p''</sub>}} and {{math|'''v'''<sub>''p''</sub>}} be elements of the tangent space {{math|''T<sub>p</sub>''(''M'')}}. The third fundamental form is then given by |
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:<math> |
:<math> |
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\ |
\mathrm{I\!I\!I}(\mathbf{u}_p,\mathbf{v}_p)=S(\mathbf{u}_p)\cdot S(\mathbf{v}_p)\,. |
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</math> |
</math> |
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==Properties== |
==Properties== |
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The third fundamental form is expressible entirely in terms of the [[first fundamental form]] and [[second fundamental form]]. If we let |
The third fundamental form is expressible entirely in terms of the [[first fundamental form]] and [[second fundamental form]]. If we let {{mvar|H}} be the mean curvature of the surface and {{mvar|K}} be the Gaussian curvature of the surface, we have |
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:<math> |
:<math> |
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\ |
\mathrm{I\!I\!I}-2H\mathrm{I\!I}+K\mathrm{I}=0\,. |
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</math> |
</math> |
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As the shape operator is self |
As the shape operator is self-adjoint, for {{math|''u'',''v'' ∈ ''T<sub>p</sub>''(''M'')}}, we find |
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:<math> |
:<math> |
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\ |
\mathrm{I\!I\!I}(u,v)=\langle Su,Sv\rangle=\langle u,S^2v\rangle=\langle S^2u,v\rangle\,. |
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</math> |
</math> |
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==See also== |
==See also== |
Latest revision as of 13:22, 13 August 2019
In differential geometry, the third fundamental form is a surface metric denoted by . Unlike the second fundamental form, it is independent of the surface normal.
Definition
[edit]Let S be the shape operator and M be a smooth surface. Also, let up and vp be elements of the tangent space Tp(M). The third fundamental form is then given by
Properties
[edit]The third fundamental form is expressible entirely in terms of the first fundamental form and second fundamental form. If we let H be the mean curvature of the surface and K be the Gaussian curvature of the surface, we have
As the shape operator is self-adjoint, for u,v ∈ Tp(M), we find
See also
[edit]