“…For a comprehensive study of the subject we refer to any of [18,19,20,21,22] and the references there. The next set of questions concern the simplicity of the spectrum and in this direction there are several papers starting from Simon [23], Jakšić-Last [24,25], Naboko-Nichols-Stolz [26], Mallick [27,28,29,30,31]. From these set of papers, we now know that when the rank of P n is one or for some special cases of higher rank P n , the singular spectrum is simple.…”