“…Such materials are being used in switches and electric surge protecting devices, among other applications. It was brought to the Oxford Study Groups with Industry in the mid 80's and has received, as a result, considerable attention in the mathematical literature, see, e.g., [1,2,3,5,7,8,9,12,15,16,19,24,25,26,28] and references therein. Related problems can be found in [21] and [27,14].…”