In differential geometry, the two principal curvatures at a given point of a surface are the maximum and minimum values of the curvature as expressed by the eigenvalues of the shape operator at that point. They measure how the surface bends by different amounts in different directions at that point.
Discussion At each point p of a differentiable surface in 3-dimensional Euclidean space one may choose a unit normal vector. A normal plane at p is one that contains the normal vector, and will therefore also contain a unique direction tangent to the surface and cut the surface in a plane curve, called normal section. This curve will in general have different curvatures for different normal planes at p. The principal curvatures at p, denoted k1 and k2, are the maximum and minimum values of this curvature. Here the curvature of a curve is by definition the reciprocal of the radius of the osculating circle. The curvature is taken to be positive if the curve turns in the same direction as the surface's chosen normal, and otherwise negative. The directions in the normal plane where the curvature takes its maximum and minimum values are always perpendicular, if k1 does not equal k2, a result of Euler (1760), and are called principal directions. From a modern perspective, this theorem follows from the spectral theorem because these directions are as the principal axes of a symmetric tensor—the second fundamental form. A systematic analysis of the principal curvatures and principal directions was undertaken by Gaston Darboux, using Darboux frames. The product k1k2 of the two principal curvatures is the Gaussian curvature, K, and the average (k1 + k2)/2 is the mean curvature, H. If at least one of the principal curvatures is zero at every point, then the Gaussian curvature will be 0 and the surface is a developable surface. For a minimal surface, the mean curvature is zero at every point.
Formal definition Let M be a surface in Euclidean space with second fundamental form I I ( X , Y ) {\displaystyle I\!I(X,Y)} . Fix a point p ∈ M, and an orthonormal basis X1, X2 of tangent vectors at p. Then the principal curvatures are the eigenvalues of the symmetric matrix
[ I I i j ] = [ I I ( X 1 , X 1 ) I I ( X 1 , X 2 ) I I ( X 2 , X 1 ) I I ( X 2 , X 2 ) ] . {\displaystyle \left[I\!I_{ij}\right]={\begin{bmatrix}I\!I(X_{1},X_{1})&I\!I(X_{1},X_{2})\\I\!I(X_{2},X_{1})&I\!I(X_{2},X_{2})\end{bmatrix}}.}
If X1 and X2 are selected so that the matrix [ I I i j ] {\displaystyle \left[I\!I_{ij}\right]} is a diagonal matrix, then they are called the principal directions. If the surface is oriented, then one often requires that the pair (X1, X2) be positively oriented with respect to the given orientation. Without reference to a particular orthonormal basis, the principal curvatures are the eigenvalues of the shape operator, and the principal directions are its eigenvectors.
Generalizations For hypersurfaces in higher-dimensional Euclidean spaces, the principal curvatures may be defined in a directly analogous fashion. The principal curvatures are the eigenvalues of the matrix of the second fundamental form I I ( X i , X j ) {\displaystyle I\!I(X_{i},X_{j})} in an orthonormal basis of the tangent space. The principal directions are the corresponding eigenvectors. Similarly, if M is a hypersurface in a Riemannian manifold N, then the principal curvatures are the eigenvalues of its second-fundamental form. If k1, ..., kn are the n principal curvatures at a point p ∈ M and X1, ..., Xn are corresponding orthonormal eigenvectors (principal directions), then the sectional curvature of M at p is given by
K ( X i , X j ) = k i k j {\displaystyle K(X_{i},X_{j})=k_{i}k_{j}}
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