- #1
Rasalhague
- 1,387
- 2
The rank of a skew-symmetric bilinear form is the minimum number of vectors in terms of which it can be expressed. We may think of a skew-symmetric bilinear form [itex]b[/itex] on [itex]V[/itex] as being in [itex]\wedge^2V^*[/itex]. If [itex]b[/itex] can be written in terms of [itex]\varepsilon^1...\varepsilon^r[/itex], then we may discard any independent [itex]\varepsilon^i[/itex]'s and extend to a basis, getting
[tex]b = b_{ij}\varepsilon^i \otimes \varepsilon^j, \enspace\enspace \text{where } b_{ij} = 0 \text{ unless } i,j \leq r,[/tex]
[tex]=b_{ij}\varepsilon^i \wedge \varepsilon^j, \enspace\enspace \text{since } b_{ij} = -b_{ji},[/tex]
so it does not matter, in the definition of rank, whether the mode of expressing [itex]b[/itex] is in terms of tensor products or exterior products.
- Bishop & Goldberg: Tensor Analysis on Manifolds, Dover 1980, §2.23, pp. 111-112
Given that Bishop and Goldberg's definition of the exterior product is
[tex]\alpha \wedge \beta := (\alpha \otimes \beta)_a[/tex]
where
[tex]\omega_a(v_1,...,v_s):=\frac{1}{s!}\sum_{(i_1,...,i_s)}\text{sgn}(i_1,...,i_s)A(v_{i_1},...v_{i_s}),[/tex]
giving, in this case,
[tex]\alpha\wedge\beta=\frac{1}{2}(\alpha\otimes\beta-\beta\otimes\alpha),[/tex]
am I right in thinking that the coefficients [itex]b_{ij}[/itex] in the quote above are not [itex]b(e_i,e_j)[/itex], but rather [itex]b_{ij} = 2b(e_i,e_j)[/itex]?
By [itex](e_i)[/itex], I mean the basis for [itex]V[/itex] with dual basis [itex](\varepsilon^i)[/itex].