Optimal codes in deletion and insertion metric
  We improve the upper bound of Levenshtein for the cardinality of a code of length 4 capable of correcting single deletions over an alphabet of even size. We also illustrate that the new upper bound is sharp. Furthermore we will construct an optimal perfect code capable of correcting single deletions for the same parameters. 
Hyun Kwang Kim, Joon Yop Lee, Dong Yeol Oh