## § Second fundamental form

- Let $z = f(x, y)$ be a (local) parametrization of the surface. Taylor expand $f$. we get:
- $f(x + dx, y + dy) = f(x, y) + dx^T a + dy^T b + dx^T L dx + 2 dx^T M dy + dy^T N dy$.
- We must get such a taylor expansion since our output is 1D (a real number), inputs are $dx, dy$ which are 3D vectors, and the infinitesimals must be linear/tensorial. These are the only possible contractions we can make.
- So, the second degree part can be written as:

$\begin{bmatrix} x & y\end{bmatrix}
\begin{bmatrix} L & M \\ M & N\end{bmatrix}
\begin{bmatrix} x \\ y\end{bmatrix}$

- the matrix in the middle, or the quadratic form $II \equiv dx^T L dx + 2 dx^T M dy + dy^T N dy$ is the second fundamental form.

#### § Classical geometry

- Let $z = f(x, y)$ be a (local) parametrization of the surface.
- At each point $p ≡ (u, v)$ on the surface within the local parametrization, we get tangent vectors $r_u(p) ≡ (\partial_x f(x, y)_p, r_v(p) ≡ (\partial_y f(x, y))_p$ which span the tangent space at $p$
- These define a unique normal vector $n(p) ≡ r_u(p) × r_v(p)$ at each point on the surface. This gives us a normal field.
- The coefficient of the second fundamental form project the second derivative of the function $f$ onto the normals. So they tell us how much the function is escaping the surface (ie, is moving along the normal to the surface) in second order.
- Recall that this is pointless to do for first order, since on a circle, tangent is perpendicular to normal, so any dot product of first order information with normal will be zero.
- Alternatively, first order information lies on tangent plane, and the normal is explicitly constructed as perpendicular to tangent plane, so any dot product of first order info with normal is zero.
- We can only really get meaningful info by dotting with normal at second order.
- So we get that $L(p) = (\partial_x \partial_x f(x, y))(p) \cdot N(p)$, $M(p) = (\partial_x \partial_y f(x, y))(p)$, and $N(p) = (\partial_y \partial_y f(x, y))(p)$, where we define $L, M, N$ via second fundamental form

#### § Proof of equivalence between 2nd fundamental form and geometry