Euler's theorem (differential geometry)

{{short description|Orthogonality of the directions of the principal curvatures of a surface}}

In the mathematical field of differential geometry, Euler's theorem is a result on the curvature of curves on a surface. The theorem establishes the existence of principal curvatures and associated principal directions which give the directions in which the surface curves the most and the least. The theorem is named for Leonhard Euler who proved the theorem in {{harv|Euler|1760}}.

More precisely, let M be a surface in three-dimensional Euclidean space, and p a point on M. A normal plane through p is a plane passing through the point p containing the normal vector to M. Through each (unit) tangent vector to M at p, there passes a normal plane PX which cuts out a curve in M. That curve has a certain curvature κX when regarded as a curve inside PX. Provided not all κX are equal, there is some unit vector X1 for which k1 = κX1 is as large as possible, and another unit vector X2 for which k2 = κX2 is as small as possible. Euler's theorem asserts that X1 and X2 are perpendicular and that, moreover, if X is any vector making an angle θ with X1, then

{{NumBlk|:|\kappa_X = k_1\cos^2\theta + k_2\sin^2\theta.\,|{{EquationRef|1}}}}

The quantities k1 and k2 are called the principal curvatures, and X1 and X2 are the corresponding principal directions. Equation ({{EquationNote|1}}) is sometimes called Euler's equation {{harv|Eisenhart|2004|p=124}}.

See also

References

  • {{citation | last = Eisenhart|first= Luther P.|authorlink=Luther Eisenhart | title=A Treatise on the Differential Geometry of Curves and Surfaces | publisher=Dover | year =2004 | isbn = 0-486-43820-1}} [https://archive.org/details/treatonthediffer00eiserich Full 1909 text] (now out of copyright)
  • {{citation|title=Recherches sur la courbure des surfaces|first=Leonhard|last=Euler|authorlink=Leonhard Euler|year=1760|journal=Mémoires de l'Académie des Sciences de Berlin|volume=16|pages=119–143|publication-date=1767|url=http://math.dartmouth.edu/~euler/pages/E333.html}}.
  • {{citation|first=Michael|last=Spivak|authorlink=Michael Spivak|title=A comprehensive introduction to differential geometry, Volume II|publisher=Publish or Perish Press|year=1999|isbn=0-914098-71-3}}

Category:Differential geometry of surfaces

Category:Theorems in differential geometry

Category:Leonhard Euler

{{differential-geometry-stub}}