Dual basis in a field extension

In mathematics, the linear algebra concept of dual basis can be applied in the context of a finite extension L/K, by using the field trace. This requires the property that the field trace TrL/K provides a non-degenerate quadratic form over K. This can be guaranteed if the extension is separable; it is automatically true if K is a perfect field, and hence in the cases where K is finite, or of characteristic zero.

A dual basis is not a concrete basis like the polynomial basis or the normal basis; rather it provides a way of using a second basis for computations.

Consider two bases for elements in a finite field, GF(pm):


 * $$B_1 = {\alpha_0, \alpha_1, \ldots, \alpha_{m-1}}$$

and


 * $$B_2 = {\gamma_0, \gamma_1, \ldots, \gamma_{m-1}}$$

then B2 can be considered a dual basis of B1 provided


 * $$\operatorname{Tr}(\alpha_i\cdot \gamma_j) = \left\{\begin{matrix} 0, & \operatorname{if}\ i \neq j\\ 1, & \operatorname{otherwise} \end{matrix}\right. $$

Here the trace of a value in GF(pm) can be calculated as follows:


 * $$\operatorname{Tr}(\beta ) = \sum_{i=0}^{m-1} \beta^{p^i}$$

Using a dual basis can provide a way to easily communicate between devices that use different bases, rather than having to explicitly convert between bases using the change of bases formula. Furthermore, if a dual basis is implemented then conversion from an element in the original basis to the dual basis can be accomplished with multiplication by the multiplicative identity (usually 1).