# Second fundamental form

Second fundamental form

In differential geometry, the second fundamental form is a quadratic form on the tangent plane of a smooth surface in the three dimensional Euclidean space, usually denoted by II. Together with the first fundamental form, it serves to define extrinsic invariants of the surface, its principal curvatures. More generally, such a quadratic form is defined for a smooth hypersurface in a Riemannian manifold and a smooth choice of the unit normal vector at each point.

Surface in R3

Motivation

The second fundamental form of a parametric surface "S" in R3 was introduced and studied by Gauss. First suppose that the surface is the graph of a twice continuously differentiable function, "z" = "f"("x","y"), and that the plane "z" = 0 is tangent to the surface at the origin. Then "f" and its partial derivatives with respect to "x" and "y" vanish at (0,0). Therefore, the Taylor expansion of "f" at (0,0) starts with quadratic terms:

: $z=Lfrac\left\{x^2\right\}\left\{2\right\} + Mxy + Nfrac\left\{y^2\right\}\left\{2\right\} + mathrm\left\{scriptstyle$ }higher{ }order{ }terms,

and the second fundamental form at the origin in the coordinates "x", "y" is the quadratic form

: $L dx^2 + 2M dx dy + N dy^2. ,$

For a smooth point "P" on "S", one can choose the coordinate system so that the coordinate "z"-plane is tangent to "S" at "P" and define the second fundamental form in the same way.

Classical notation

The second fundamental form of a general parametric surface is defined as follows. Let r=r("u","v") be a regular parametrization of a surface in R3, where r is a smooth vector valued function of two variables. It is common to denote the partial derivatives of r with respect to "u" and "v" by ru and rv. Regularity of the parametrization means that ru and rv are linearly independent for any ("u","v") in the domain of r, and hence span the tangent plane to "S" at each point. Equivalently, the cross product ru &times; rv is a nonzero vector normal to the surface. The parametrization thus defines a field of unit normal vectors n:

:$mathbf\left\{n\right\} = frac\left\{mathbf\left\{r\right\}_u imesmathbf\left\{r\right\}_v\right\}.$

The second fundamental form is usually written as

:$mathrm\left\{II\right\} = Ldu^2 + 2Mdudv + Ndv^2, ,$

its matrix in the basis {ru, rv} of the tangent plane is

:

The coefficients "L", "M", "N" at a given point in the parametric "uv"-plane are given by the projections of the second partial derivatives of r at that point onto the normal line to "S" and can be computed with the aid of the dot product as follows:

:$L = mathbf\left\{r\right\}_\left\{uu\right\} cdot mathbf\left\{n\right\}, quadM = mathbf\left\{r\right\}_\left\{uv\right\} cdot mathbf\left\{n\right\}, quadN = mathbf\left\{r\right\}_\left\{vv\right\} cdot mathbf\left\{n\right\}.$

Modern notation

The second fundamental form of a general parametric surface "S" is defined as follows: Let r=r("u"1,"u"2) be a regular parametrization of a surface in R3, where r is a smooth vector valued function of two variables. It is common to denote the partial derivatives of r with respect to "u"α by rα, α = 1, 2. Regularity of the parametrization means that r1 and r1 are linearly independent for any ("u"1,"u"2) in the domain of r, and hence span the tangent plane to "S" at each point. Equivalently, the cross product r1 &times; r2 is a nonzero vector normal to the surface. The parametrization thus defines a field of unit normal vectors n:

:$mathbf\left\{n\right\} = frac\left\{mathbf\left\{r\right\}_1 imesmathbf\left\{r\right\}_2\right\}.$

The second fundamental form is usually written as

:

The equation above implies Einstein Summation Convention. The coefficients "b"α,β at a given point in the parametric ("u"1, "u"2)-plane are given by the projections of the second partial derivatives of r at that point onto the normal line to "S" and can be computed with the aid of the dot product as follows:

:

Hypersurface in a Riemannian manifold

In Euclidean space, the second fundamental form is given by

:$I!I\left(v,w\right) = langle d u\left(v\right),w angle$

where $u$ is the Gauss map, and $d u$ the differential of $u$ regarded as a vector valued differential form, and the brackets denote the metric tensor of Euclidean space.

More generally, on a Riemannian manifold, the second fundamental form is an equivalent way to describe the shape operator (denoted by "S") of a hypersurface,

:$mathrm I!mathrm I\left(v,w\right)=langle S\left(v\right),w angle= -langle abla_v n,w angle=langle n, abla_v w angle,$

where $abla_v w$ denotes the covariant derivative of the ambient manifold and "n" a field of normal vectors on the hypersurface. (If the affine connection is torsion-free, then the second fundamental form is symmetric.)

The sign of the second fundamental form depends on the choice of direction of "n" (which is called a co-orientation of the hypersurface - for surfaces in Euclidean space, this is equivalently given by a choice of orientation of the surface).

Generalization to arbitrary codimension

The second fundamental form can be generalized to arbitrary codimension. In that case it is a quadratic form on the tangent space with values in the normal bundle and it can be defined by

:

where denotes the orthogonal projection of covariant derivative $abla_v w$ onto the normal bundle.

In Euclidean space, the curvature tensor of a submanifold can be described by the following formula:

:$langle R\left(u,v\right)w,z angle =langle mathrm I!mathrm I\left(u,z\right),mathrm I!mathrm I\left(v,w\right) angle-langle mathrm I!mathrm I\left(u,w\right),mathrm I!mathrm I\left(v,z\right) angle.$

This is called the Gauss equation, as it may be viewed as a generalization of Gauss's Theorema Egregium. The eigenvalues of the second fundamental form, represented in an orthonormal basis, are the principal curvatures of the surface. A collection of orthonormal eigenvectors are called the principal directions.

For general Riemannian manifolds one has to add the curvature of ambient space; if "N" is a manifold embedded in a Riemannian manifold ("M,g") then the curvature tensor $R_N$ of "N" with induced metric can be expressedusing the second fundamental form and $R_M$, the curvature tensor of "M":

:$langle R_N\left(u,v\right)w,z angle = langle R_M\left(u,v\right)w,z angle+langle mathrm I!mathrm I\left(u,z\right),mathrm I!mathrm I\left(v,w\right) angle-langle mathrm I!mathrm I\left(u,w\right),mathrm I!mathrm I\left(v,z\right) angle.$

ee also

*First fundamental form
*Gaussian curvature
*Gauss–Codazzi equations

References

*
*cite book | author=Kobayashi, Shoshichi and Nomizu, Katsumi | title = Foundations of Differential Geometry, Vol. 2 | publisher=Wiley-Interscience | year=1996 (New edition) |id = ISBN 0471157325
*

* A PHD thesis about the geometry of the second fundamental form by Steven Verpoort: https://repository.libis.kuleuven.be/dspace/bitstream/1979/1779/2/hierrrissiedan!.pdf

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