Let sn be the number of topics reviewed in the previous n days * * *, which is obviously a positive integer that increases strictly, that is, each topic is not repeated and the maximum value does not exceed11x12 =132. Consider the sequence an=sn, bn=sn+2 1. Where an has 1 1x7=77 items, bn has 77 items, and * * * has 154 positive integers. And these numbers cannot exceed11x12+21=153 at most. The pigeon hole principle knows that there must be repeated numbers, but an and bn are not repeated, so there is am=bn, that is, sm=sn+2 1. In other words, the number of topics reviewed from the n+ 1 day to the m-day is exactly 2 1.