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Dual code : ウィキペディア英語版
Dual code

In coding theory, the dual code of a linear code
:C\subset\mathbb_q^n
is the linear code defined by
:C^\perp = \
where
:\langle x, c \rangle = \sum_^n x_i c_i
is a scalar product. In linear algebra terms, the dual code is the annihilator of ''C'' with respect to the bilinear form <,>. The dimension of ''C'' and its dual always add up to the length ''n'':
:\dim C + \dim C^\perp = n.
A generator matrix for the dual code is a parity-check matrix for the original code and vice versa. The dual of the dual code is always the original code.
==Self-dual codes==
A self-dual code is one which is its own dual. This implies that ''n'' is even and dim ''C'' = ''n''/2. If a self-dual code is such that each codeword's weight is a multiple of some constant c > 1, then it is of one of the following four types:
*Type I codes are binary self-dual codes which are not doubly even. Type I codes are always even (every codeword has even Hamming weight).
*Type II codes are binary self-dual codes which are doubly even.
*Type III codes are ternary self-dual codes. Every codeword in a Type III code has Hamming weight divisible by 3.
*Type IV codes are self-dual codes over F4. These are again even.
Codes of types I, II, III, or IV exist only if the length ''n'' is a multiple of 2, 8, 4, or 2 respectively.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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