Perfect codes have been introduced in §6.5 in connection with the Hamming bound (6.5.1) which for a q-ary code C = (n, k, d) with d ≤ 2t + 1 implies that
) (q−1)i ≤ qn, (7.1.1)
where |C| (= M) denotes the size of the code C (see §6.1) A code is said to be perfect if equality holds in (7.1.1). Since a t-error correcting code can correct up to t errors, a perfect t-error correcting code must be such that every codeword lies within a distance of t to exactly one codeword. In other words, if the code has dmin = 2t+ 1 that covers radius t, where the covering radius has the smallest number, then every codeword lies within a distance of the Hamming radius t to a codeword. In the case of a perfect code, the Hamming bound holds (see §6.1.4), which leads to a rigorous definition of a perfect code: If there is an (n, k) code with an alphabet of q elements, and if dmin = 2t+ 1, then the inequality (6.5.2) holds.