Dual additive code

Description

For any \(q\)-ary additive code \(C\), the dual code is the set of \(q\)-ary strings that are orthogonal to the codewords of \(C\) under a particular inner product.

The dual additive (or orthogonal additive) code is \begin{align} C^\perp = \{ y\in GF(q)^{n} ~|~ x \star y=0 \forall x\in C\}, \tag*{(1)}\end{align} where the trace inner product is \(x\star y = \sum_{i=1}^n \text{tr}(x_i y_i)\) for coordinates \(x_i,y_i\) and for \(\textit{tr}\) being the field trace.

A code that is contained in its dual, \(C \subseteq C^\perp\), is called self-orthogonal additive or weakly self-dual additive. A code that contains its dual, \(C^\perp \subseteq C\), is called dual-containing additive. A code that is equal to its dual, \(C^\perp = C\), is called self-dual additive. A code is dual-containing additive iff its dual is self-orthogonal additive.

An alternative definition of dual substitutes the trace inner product for the trace-Hermitian inner product, \(x\star y \to \sum_{i=1}^n \text{tr}(x_i y^{p}_i)\). Another extension for when \(q=p^2\), relevant to certain stabilizer codes and reducing to the trace-Hermitian case for \(q=4\), is the trace-alternating inner product, \begin{align} x\star y \to \sum_{i=1}^{n}\text{tr}\left(\frac{x_{i}y_{i}^{\sqrt{q}}-x_{i}^{\sqrt{q}}y_{i}}{\alpha-\alpha^{q}}\right)~, \tag*{(2)}\end{align} where \(\{1,\alpha\}\) is a basis of \(GF(q)\) over \(GF(\sqrt{q})\). Self-dual additive codes with respect to the trace-Hermitian (trace-alternating) inner product are called trace Hermitian (trace-alternating) self-dual additive; similar definitions hold for self-orthogonal additive and dual-containing additive.