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dual code (Definition)

Let $ C$ be a linear code of block length $ n$ over the finite field $ \mathbb{F}_q$. Then the set

$\displaystyle C^\perp:=\{d\in\mathbb{F}_q^n\mid c\cdot d=0$ for all $\displaystyle c\in C\}$    

is the dual code of $ C$. Here, $ c\cdot d$ denotes either the standard dot product or the Hermitian dot product.

This definition is reminiscent of orthogonal complements of finite dimensional vector spaces over the real or complex numbers. Indeed, $ C^\perp$ is also a linear code and it is true that if $ k$ is the dimension of $ C$, then the dimension of $ C^\perp$ is $ n-k$. It is, however, not necessarily true that $ C\cap C^\perp=\{0\}$. For example, if $ C$ is the binary code of block length $ 2$ spanned by the codeword $ (1,1)$ then $ (1,1)\cdot(1,1)=0$, that is, $ (1,1)\in C^\perp$. In fact, $ C$ equals $ C^\perp$ in this case. In general, if $ C=C^\perp$, $ C$ is called self-dual. Furthermore $ C$ is called self-orthogonal if $ C\subseteq C^\perp$.

Famous examples of self-dual codes are the extended binary Hamming code of block length $ 8$ and the extended binary Golay code of block length $ 24$.



"dual code" is owned by GrafZahl.
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See Also: linear code, orthogonal complement

Also defines:  self-dual, self-orthogonal
Keywords:  linear code, orthogonal complement
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Cross-references: extended binary golay code, binary, codes, binary code, complex numbers, real, vector spaces, orthogonal complements, Hermitian dot product, dot product, finite field, block length, linear code
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This is version 3 of dual code, born on 2005-05-01, modified 2005-05-13.
Object id is 6989, canonical name is DualCode.
Accessed 3992 times total.

Classification:
AMS MSC94B05 (Information and communication, circuits :: Theory of error-correcting codes and error-detecting codes :: Linear codes, general)

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