Exercise 2.14 In bellringing, successive permutations of n bells are played one after the other. Following one permutation T, the next permutation must be obtained from π by moving the position of each bell by at most one place. For example, for n = 4, the permutation 1234 could be followed by any one of 2134, 2143, 1324, 1243. Show that Ian denotes the number of permutations which could follow 12...n, then an = an-1 + an-2 +1. Hence find an.