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ADVANCED MATH
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Two surfaces S and $\overline{S}$, with a common point p, have contact of order $\geq 2$ at p if there exist parametrizations $\mathbf{x}(u, v)$ and $\overline{\mathbf{x}}(u, v)$ in p of S and $\overline{S},$ respectively, such that $$ \mathbf{x}_{u}=\overline{\mathbf{x}}_{u}, \quad \mathbf{x}_{v}=\overline{\mathbf{x}}_{v}, \quad \mathbf{x}_{u u}=\overline{\mathbf{x}}_{u u}, \quad \mathbf{x}_{u v}=\overline{\mathbf{x}}_{u v}, \quad \mathbf{x}_{v v}=\overline{\mathbf{x}}_{v v} $$ at p. Prove the following: a. Let S and $\bar{S}$ have contact of order $\geq 2$ at $p; \mathbf{x}: U \longrightarrow S$ and $\overline{\mathbf{x}}: U \longrightarrow \bar{S}$ be arbitrary parametrizations in p of S and $\bar{S}$, respectively; and $f: V \subset R^{3} \rightarrow R$ be a differentiable function in a neighborhood V of p in $R^{3}$. Then the partial derivatives of order $\leq 2$ of $f \circ \overline{\mathbf{x}}: U \rightarrow R$ are zero in $\overline{\mathbf{x}}^{-1}(p)$ if and only if the partial derivatives of order $\leq 2$ of $f \circ \mathbf{x}: U \longrightarrow R$ are zero in $\mathbf{x}^{-1}(p)$. b. Let S and $\bar{S}$ have contact of order $\geq 2$ at p. Let $z=f(x, y), z=\overline{f}(x, y)$ be the equations, in a neighborhood of p, of S and $\bar{S}$, respectively, where the xy plane is the common tangent plane at p = (0, 0). Then the function $f(x, y)-\overline{f}(x, y)$ has all partial derivatives of order $\leq 2,$ at (0, 0), equal to zero. c. Let p be a point in a surface $S \subset R^{3}$. Let Oxyz be a cartesian coordinate system for $R^{3}$ such that v = p and the xy plane is the tangent plane of S at p. Show that the paraboloid $$ z=\frac{1}{2}\left(x^{2} f_{x x}+2 x y f_{x y}+y^{2} f_{y y}\right), \quad (*) $$ obtained by neglecting third- and higher-order terms in the Taylor development around p = (0, 0), has contact of order $\geq 2$ at p with S (the surface (*) is called the osculating paraboloid of S at p). d. If a paraboloid (the degenerate cases of plane and parabolic cylinder are included) has contact of order $\geq 2$ with a surface S at p, then it is the osulating paraboloid of S at p. e. If two surfaces have contact of order $\geq 2$ at p, then the osulating parabolois of S and $\bar{S}$ are coincide . Conclude that the Gaussian and mean curvatures of S and $\bar{S}$ are equal. f. The notion of contact of order $\geq 2$ is invariant by diffeomorphisms of $R^{3}$; that is, if S and $\bar{S}$ have contact of order $\geq 2$ at p and $\varphi: R^{3} \rightarrow R^{3}$ is a diffeomorphism, then $\varphi(S)$ and $\varphi(\bar{S})$ have contact of order $\geq 2$ at $\varphi(p)$. g. If S and $\bar{S}$ have contact of order $\geq 2$ at p, then $$ \lim _{r \rightarrow 0} \frac{d}{r^{2}}-0, $$ where d is the length of the segment cut by the surfaces in a straight line normal to $T_{p}(S)=T_{p}(\bar{S}),$ which is at a distance r from p.
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