Perfect code
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Let C be an error-correcting code consisting of N codewords,in which each codeword consists of n letters taken from an alphabet A of length q, and every two distinct codewords differ in at least d==2e+1 places. Then C is said to be perfect if for every possible word w_0 of length n with letters in A, there is a unique code word w in C in which at most e letters of w differ from the corresponding letters of w_0.
It is straightforward to show that C is perfect if sum_(i==0)^e(n; i)(q-1)^i==(q^n)/N.
If C is a binary linear code, then q==2 and N==2^k, where k is the number of generators of C, in which case C is perfect if sum_(i==0)^e(n; i)==2^(n-k).
Hamming codes and the Golay code are examples of perfect codes.