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Calculus of moving surfaces : ウィキペディア英語版
Calculus of moving surfaces

The calculus of moving surfaces (CMS) 〔Grinfeld, P. (2010). "Hamiltonian Dynamic Equations for Fluid Films". Studies in Applied Mathematics. . ISSN 00222526.〕 is an extension of the classical tensor calculus to deforming manifolds. Central to the CMS is the \delta /\delta t-derivative whose original definition 〔J. Hadamard, Lecons Sur La Propagation Des Ondes et Les Equations De
l’Hydrodynamique. Paris: Hermann, 1903.〕 was put forth by Jacques Hadamard. It plays the role analogous to that of the covariant derivative \nabla _ on differential manifolds. In particular, it has the property that it produces a tensor when applied to a tensor.
Suppose that S_t is the evolution of the surface S indexed by a time-like parameter t. The definitions of the surface velocity C and the operator \delta /\delta t are the geometric foundations of the CMS. The velocity C is the rate of deformation of the surface S in the instantaneous normal direction. The value of C at a point P is defined as the limit
: C=\lim_ \frac
where P^ is the point on S_ that lies on the straight line perpendicular to S_ at point P. This definition is illustrated in the first geometric figure below. The velocity C is a signed quantity: it is positive when \overline and C is analogous to the relationship between location and velocity in elementary calculus: knowing either quantity allows one to construct the other by differentiation or integration.
The \delta /\delta t-derivative for a scalar field F defined on S_ is the rate of change in F in the instantaneously normal direction:
: \frac=\lim_ \frac
This definition is also illustrated in second geometric figure.
The above definitions are ''geometric''. In analytical settings, direct application of these definitions may not be possible. The CMS gives ''analytical'' definitions of C and \delta /\delta t in terms of elementary operations from calculus and differential geometry.
==Analytical definitions==

For analytical definitions of C and \delta /\delta t, consider the evolution of S given by
: Z^i = Z^i \left( t ,S \right) \,
where Z^ are general curvilinear space coordinates and S^ are the surface coordinates. By convention, tensor indices of function arguments are dropped. Thus the above equations contains S rather than S^\alpha.The velocity object
v^ is defined as the partial derivative
: v^i =\frac
The velocity C can be computed most directly by the formula
: C=v^i N_i \,
where N_i are the covariant components of the normal vector \vec.
The definition of the \delta /\delta t-derivative for an invariant ''F'' reads
: \frac=\frac-v^Z^_\nabla _F
where Z^\alpha_i is the shift tensor and
\nabla_\alpha is the covariant derivative on S.
For ''tensors'', an appropriate generalization is needed. The proper definition for a representative tensor T^_ reads
: \frac}=\frac}-v^\nabla _T^_+v^\Gamma ^_T^_-v^\Gamma ^_T^_+\nabla _v^T^_-\nabla _v^T^_
where \Gamma^k_ are Christoffel symbols.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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