# Second fundamental form

In differential geometry, the second fundamental form (or shape tensor) is a quadratic form on the tangent plane of a smooth surface in the three-dimensional Euclidean space, usually denoted by ${\displaystyle \mathrm {I\!I} }$ (read "two"). 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

Definition of second fundamental form

### 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:

${\displaystyle z=L{\frac {x^{2}}{2}}+Mxy+N{\frac {y^{2}}{2}}+\mathrm {\scriptstyle {{\ }higher{\ }order{\ }terms}} ,}$

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

${\displaystyle L\,{\text{d}}x^{2}+2M\,{\text{d}}x\,{\text{d}}y+N\,{\text{d}}y^{2}.\,}$

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 × rv is a nonzero vector normal to the surface. The parametrization thus defines a field of unit normal vectors n:

${\displaystyle \mathbf {n} ={\frac {\mathbf {r} _{u}\times \mathbf {r} _{v}}{|\mathbf {r} _{u}\times \mathbf {r} _{v}|}}.}$

The second fundamental form is usually written as

${\displaystyle \mathrm {I\!I} =L\,{\text{d}}u^{2}+2M\,{\text{d}}u\,{\text{d}}v+N\,{\text{d}}v^{2},\,}$

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

${\displaystyle {\begin{bmatrix}L&M\\M&N\end{bmatrix}}.}$

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:

${\displaystyle L=\mathbf {r} _{uu}\cdot \mathbf {n} ,\quad M=\mathbf {r} _{uv}\cdot \mathbf {n} ,\quad N=\mathbf {r} _{vv}\cdot \mathbf {n} .}$

### Physicist's notation

The second fundamental form of a general parametric surface S is defined as follows: Let r=r(u1,u2) 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 uα by rα, α = 1, 2. Regularity of the parametrization means that r1 and r2 are linearly independent for any (u1,u2) in the domain of r, and hence span the tangent plane to S at each point. Equivalently, the cross product r1 × r2 is a nonzero vector normal to the surface. The parametrization thus defines a field of unit normal vectors n:

${\displaystyle \mathbf {n} ={\frac {\mathbf {r} _{1}\times \mathbf {r} _{2}}{|\mathbf {r} _{1}\times \mathbf {r} _{2}|}}.}$

The second fundamental form is usually written as

${\displaystyle \mathrm {I\!I} =b_{\alpha \beta }\,{\text{d}}u^{\alpha }\,{\text{d}}u^{\beta }.\,}$

The equation above uses the Einstein Summation Convention. The coefficients bαβ at a given point in the parametric (u1, u2)-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 in terms of the normal vector "n" as follows:

${\displaystyle b_{\alpha \beta }=r_{,\alpha \beta }^{\ \ \gamma }n_{\gamma }.}$

## Hypersurface in a Riemannian manifold

In Euclidean space, the second fundamental form is given by

${\displaystyle \mathrm {I\!I} (v,w)=-\langle d\nu (v),w\rangle \nu }$

where ${\displaystyle \nu }$ is the Gauss map, and ${\displaystyle d\nu }$ the differential of ${\displaystyle \nu }$ 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 ${\displaystyle S}$) of a hypersurface,

${\displaystyle \mathrm {I} \!\mathrm {I} (v,w)=\langle S(v),w\rangle n=-\langle \nabla _{v}n,w\rangle n=\langle n,\nabla _{v}w\rangle n,}$

where ${\displaystyle \nabla _{v}w}$ denotes the covariant derivative of the ambient manifold and ${\displaystyle n}$ 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 ${\displaystyle n}$ (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

${\displaystyle \mathrm {I\!I} (v,w)=(\nabla _{v}w)^{\bot },}$

where ${\displaystyle (\nabla _{v}w)^{\bot }}$ denotes the orthogonal projection of covariant derivative ${\displaystyle \nabla _{v}w}$ onto the normal bundle.

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

${\displaystyle \langle R(u,v)w,z\rangle =\langle \mathrm {I} \!\mathrm {I} (u,z),\mathrm {I} \!\mathrm {I} (v,w)\rangle -\langle \mathrm {I} \!\mathrm {I} (u,w),\mathrm {I} \!\mathrm {I} (v,z)\rangle .}$

This is called the Gauss equation, as it may be viewed as a generalization of Gauss's Theorema Egregium.

For general Riemannian manifolds one has to add the curvature of ambient space; if ${\displaystyle N}$ is a manifold embedded in a Riemannian manifold (${\displaystyle M,g}$) then the curvature tensor ${\displaystyle R_{N}}$ of ${\displaystyle N}$ with induced metric can be expressed using the second fundamental form and ${\displaystyle R_{M}}$, the curvature tensor of ${\displaystyle M}$:

${\displaystyle \langle R_{N}(u,v)w,z\rangle =\langle R_{M}(u,v)w,z\rangle +\langle \mathrm {I} \!\mathrm {I} (u,z),\mathrm {I} \!\mathrm {I} (v,w)\rangle -\langle \mathrm {I} \!\mathrm {I} (u,w),\mathrm {I} \!\mathrm {I} (v,z)\rangle .}$