Double-Change Covering Designs with Block Size K =

A double-change covering design (dccd) is an ordered set of blocks with block size k is an ordered collection of b blocks, B = {B 1, B2, ... , Bb}, each an unordered subset of k distinct elements from [v] = {1, 2, ... , v}, which obey: (1) each block differs from the previous block by two elements, and, (2) every unordered pair of [v] appears in at least one block. The object is to minimize b for a fixed v and k. Tight designs are those in which each pair is covered exactly once. We present constructions of tight dccd's for arbitrary v when k = 2 and minimal constructions for v ≤ 20 when k = 4. A general, but not minimal, method is presented to construct circular dccd for arbitrary v when k = 4.