The next technique that Cartan employed in his book on Lecons on Integraux
Invariants is the concept of the exterior derivative. This concept is a rule of
differentiation that transports an element of a lower dimensional exterior
algebra subspace to the next higher dimensional exterior algebra subspace. The
primitive example is the use of the gradient operator acting on a function to
construct a gradient vector field. The gradient operation takes and element of
(n,0) into an element of (n,1).
What is remarkable is that the operational definition of the exterior derivative
works on any p-form carrying it into a p+1 form. For example, the exterior
derivative of an p=N-1 form carries it into the a p = N form.
In engineering technology, one would say the the divergence of a N-1 current
(vector) produces a density. IN N=3 dimensions, the Pascal triangle of
alegbraic exterior algebra subspaces has dimensions, {1,3,3,1}. In engineering
practice it is recognized that the differential operations defined as the "Curl"
takes a 3-component vector from (n,1) into another 3-component (psuedo) vector
of the space (n,2). These two vectors have the same number of components only
in a space of N=3 dimensions (that is why the Gibbs cross product is useful).
However the two species of vectors are never physicaly equivalent. No engineer
ever adds linear momentum to angular momentum. No engineer ever adds a Magnetic
field 3 component vector to an electric field 3 component vector. The concept
of the gradient and the concept of the curl are treated differently in Gibbsian
vector analysis.
In the Cartan exterior calculus, the operations are the same and are represented
by the same exterior derivative, but the operation operates on different
algebraic subspaces.
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