 Define the following distance function formula_14 on the words formula_15 of length "n" transmitted via a Z-channel
Define the sphere formula_17 of radius "t" around a word formula_18 of length "n" as the set of all the words at distance "t" or less from formula_19, in other words,
A code formula_21 of length "n" is said to be "t"-asymmetric-error-correcting if for any two codewords formula_22, one has formula_23.Denote by formula_24 the maximum number of codewords in a "t"-asymmetric-error-correcting code of length "n".The Varshamov bound.