- Gaussian curvature
In

differential geometry , the**Gaussian curvature**or**Gauss curvature**of a point on asurface is the product of theprincipal curvature s, "κ"_{1}and "κ"_{2}, of the given point. It is an "intrinsic" measure of curvature, i.e., its value depends only on how distances are measured on the surface, not on the way it is embedded in space. This result is the content of Gauss'sTheorema egregium .Symbolically, the Gaussian

curvature "Κ" is defined as:$Kappa\; =\; kappa\_1\; kappa\_2\; ,!$.It is also given by: $Kappa\; =\; frac\{langle\; (\; abla\_2\; abla\_1\; -\; abla\_1\; abla\_2)mathbf\{e\}\_1,\; mathbf\{e\}\_2\; angle\}\{det\; g\},$where $abla\_i\; =\; abla\_$mathbf e}_i} is the

covariant derivative and "g" is themetric tensor .At a point

**p**on a regular surface in**R**^{"3"}, the Gaussian curvature is also given by: $K(mathbf\{p\})\; =\; det(S(mathbf\{p\})),$where "S" is theshape operator .A useful formula for the Gaussian curvature is Liouville's equation in terms of the Laplacian in

isothermal coordinates .**Informal definition**We represent the surface by the implicit function theorem as the graph of a function f of 2 variables, and assume the point p is a critical point, i.e. the gradient of f vanishes (this can always be attained by a suitable rigid motion). Then the Gaussian curvature of the surface at p is the determinant of the Hessian matrix of f, i.e. the 2 by 2 matrix of second derivatives. This definition allows one immediately to grasp the distinction between cup/cap

*versus*saddle point behavior in terms of second year calculus.**Total curvature**The

surface integral of the Gaussian curvature over some region of a surface is called the**total curvature**. The total curvature of ageodesic triangle equals the deviation of the sum of its angles from $pi$. The sum of the angles of a triangle on a surface of positive curvature will exceed $pi$, while the sum of the angles of a triangle on a surface of negative curvature will be less than $pi$. On a surface of zero curvature, such as theEuclidean plane , the angles will sum to precisely $pi$.:$sum\_\{i=1\}^3\; heta\_i\; =\; pi\; +\; iint\_T\; K\; ,dA.$

A more general result is the

Gauss-Bonnet Theorem .**Important theorems****Theorema egregium**Gauss's

**Theorema Egregium**(Latin: "remarkable theorem") states that Gaussian curvature of a surface can be determined from the measurements of length on the surface itself. In fact, it can be found given the full knowledge of thefirst fundamental form and expressed via the first fundamental form and itspartial derivative s of first and second order. Equivalently, thedeterminant of thesecond fundamental form of a surface in**R**^{3}can be so expressed. The "remarkable", and surprising, feature of this theorem is that although the "definition" of the Gaussian curvature of a surface "S" in**R**^{3}certainly depends on the way in which the surface is located in space, the end result, the Gaussian curvature itself, is determined by the inner metric of the surface without any further reference to the ambient space: it is anintrinsic invariant. In particular, the Gaussian curvature is invariant under isometric deformations of the surface.In contemporary

differential geometry , a "surface", viewed abstractly, is a two-dimensionaldifferentiable manifold . To connect this point of view with the classical theory of surfaces, such an abstract surface is embedded into**R**^{3}and endowed with theRiemannian metric given by the first fundamental form. Suppose that the image of the embedding is a surface "S" in**R**^{3}. A "local isometry" is adiffeomorphism "f": "U" → "V" between open regions of**R**^{3}whose restriction to "S" ∩ "U" is anisometry onto its image.**Theorema Egregium**is then stated as follows:: The Gaussian curvature of an embedded smooth surface in

**R**^{3}is invariant under the local isometries.For example, the Gaussian curvature of a cylindrical tube is zero, the same as for the "unrolled" tube (which is flat). [

*Porteous, I. R., "Geometric Differentiation". Cambridge University Press, 1994. ISBN 0-521-39063-X*] On the other hand, since asphere of radius "R" has constant positive curvature "R"^{−2}and a flat plane has constant curvature 0, these two surfaces are not isometric, even locally. Thus any planar representation of even a part of a sphere must distort the distances. Therefore, nocartographic projection is perfect.**Gauss–Bonnet theorem**The Gauss-Bonnet theorem links the total curvature of a surface to its

Euler characteristic and provides an important link between local geometric properties and global topological properties.**Surfaces of constant curvature***

**Minding's theorem**(1839) states that all surfaces with the same constant curvature "K" are locally isometric. A consequence of Minding's theorem is that any surface whose curvature is identically zero can be constructed by bending some plane region. Such surfaces are calleddevelopable surface s. Minding also raised the question whether aclosed surface with constant positive curvature is necessarily rigid.*

**Liebmann's theorem**(1900) answered Minding's question. The only regular (of class "C"^{2}) closed surfaces in**R**^{3}with constant positive Gaussian curvature aresphere s. [*cite book | last = Kühnel | first = Wolfgang | title = Differential Geometry: Curves - Surfaces - Manifolds | publisher = American Mathematical Society | date = 2006 | id = ISBN 0821839888*]*

**Hilbert's theorem**(1901) states that there exists no complete analytic (class "C"^{"ω"}) regular surface in**R**^{3}of constant negative Gaussian curvature. In fact, the conclusion also holds for surfaces of class "C"^{2}immersed in**R**^{3}, but breaks down for "C"^{1}-surfaces. Thepseudosphere has constant negative Gaussian curvature except at its singular cusp. [*[*]*http://eom.springer.de/h/h047410.htm "Hilbert theorem". Springer Online Reference Works.*]**Alternative Formulas***Gaussian curvature of a surface in

**R**^{3}can be expressed as the ratio of thedeterminant s of the second and first fundamental forms::$K\; =\; frac\{det\; II\}\{det\; I\}\; =\; frac\{LN-M^2\}\{EG-F^2\}.$

*The

**Brioschi formula**gives Gaussian curvature solely in terms of the first fundamental form::$K\; =\; left(\; egin\{vmatrix\}\; -frac\{1\}\{2\}E\_\{vv\}\; +\; F\_\{uv\}\; -\; frac\{1\}\{2\}G\_\{uu\}\; frac\{1\}\{2\}E\_u\; F\_u-frac\{1\}\{2\}E\_v\backslash F\_v-frac\{1\}\{2\}G\_u\; E\; F\backslash frac\{1\}\{2\}G\_v\; F\; G\; end\{vmatrix\}-egin\{vmatrix\}\; 0\; frac\{1\}\{2\}E\_v\; frac\{1\}\{2\}G\_u\backslash frac\{1\}\{2\}E\_v\; E\; F\backslash frac\{1\}\{2\}G\_u\; F\; G\; end\{vmatrix\}\; ight)\; /\; ,\; (EG-F^2)^2$*For an

**orthogonal parametrization**, Gaussian curvature is::$K\; =\; -frac\{1\}\{2sqrt\{EG$left(frac{partial}{partial u}frac{G_u}{sqrt{EG + frac{partial}{partial v}frac{E_v}{sqrt{EG ight).*Gaussian curvature is the limiting difference between the

and a circle in the plane::$K\; =\; lim\_\{r\; arr\; 0\}\; (2\; pi\; r\; -\; mbox\{C\}(r))\; cdot\; frac\{3\}\{pi\; r^3\}$circumference of a geodesic circle*Gaussian curvature is the limiting difference between the

and a circle in the plane::$K\; =\; lim\_\{r\; arr\; 0\}\; (pi\; r^2\; -\; mbox\{A\}(r))\; cdot\; frac\{12\}\{pi\; r^4\}$area of a geodesic circle*Gaussian curvature may be expressed with the

: [Christoffel symbols *cite book | last = Struik | first = Dirk| title = Lectures on Classical Differential Geometry | publisher = Courier DoverPublications | date = 1988 | id = ISBN 0486656098*] :$K\; =\; -frac\{1\}\{E\}\; left(\; frac\{partial\}\{partial\; u\}Gamma\_\{12\}^2\; -\; frac\{partial\}\{partial\; v\}Gamma\_\{11\}^2\; +\; Gamma\_\{12\}^1Gamma\_\{11\}^2\; -\; Gamma\_\{11\}^1Gamma\_\{12\}^2\; +\; Gamma\_\{12\}^2Gamma\_\{12\}^2\; -\; Gamma\_\{11\}^2Gamma\_\{22\}^2\; ight)$**References****ee also***

sectional curvature

*Mean curvature

*Theorema egregium

*Gauss map

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