Selecting two different defective coins

In this paper, given a balance scale and the information that there are exactly two different defective coins present, the authors consider the problem of ascertaining the minimum number of testing which suffice to determine the two different defective coins in a set of λ coins in same appearance, and here λ ⩾ 3. A testing algorithm for all the possible values of λ is constructed, and the testing algorithm needs at most one testing step more than the optimal testing algorithm.