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Geometric calculus : ウィキペディア英語版
Geometric calculus

In mathematics, geometric calculus extends the geometric algebra to include differentiation and integration. The formalism is powerful and can be shown to encompass other mathematical theories including differential geometry and differential forms.〔David Hestenes, Garrett Sobczyk: Clifford Algebra to Geometric Calculus, a Unified Language for mathematics and Physics (Dordrecht/Boston:G.Reidel Publ.Co., 1984, ISBN 90-277-2561-6〕
==Differentiation==

With a geometric algebra given, let ''a'' and ''b'' be vectors and let ''F(a)'' be a multivector-valued function. The directional derivative of ''F(a)'' along ''b'' is defined as
:\nabla_b F(a) = \lim_}
provided that the limit exists, where the limit is taken for scalar ''ε''. This is similar to the usual definition of a directional derivative but extends it to functions that are not necessarily scalar-valued.
Next, choose a set of basis vectors \ and consider the operators, noted (\partial_i), that perform directional derivatives in the directions of (e_i):
:\partial_i : F \mapsto (x\mapsto \nabla_ F(x))
Then, using the Einstein summation notation, consider the operator :
:e^i\partial_i
which means:
:F \mapsto e^i\partial_i F
or, more verbosely:
:F \mapsto (x\mapsto e^i\nabla_ F(x))
It can be shown that this operator is independent of the choice of frame, and can thus be used to define the ''geometric derivative'':
:\nabla = e^i\partial_i
This is similar to the usual definition of the gradient, but it, too, extends to functions that are not necessarily scalar-valued.
It can be shown that the directional derivative is linear regarding its direction, that is:
:\nabla_ = \alpha\nabla_a + \beta\nabla_b
From this follows that the directional derivative is the inner product of its direction by the geometric derivative. All needs to be observed is that the direction a can be written a = (a\cdot e^i) e_i, so that:
:\nabla_a = \nabla_ = (a\cdot e^i)\nabla_ = a\cdot(e^i\nabla_) = a\cdot \nabla
For this reason, \nabla_a F(x) is often noted a\cdot \nabla F(x).
The standard order of operations for the geometric derivative is that it acts only on the function closest to its immediate right. Given two functions ''F'' and ''G'', then for example we have
:\nabla FG = (\nabla F)G.

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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