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题目(2006年湖南,文20)在m(m≥2)个不同数的排列p1p2…pm中,若1≤ipj(即前面某数大于后面某数),则称pi与pj构成一个逆序．一个排列的全部逆序的总数称为该排列的逆序数．记排列(n+1)n(n-1)…321的逆序数为an,如排列21的逆序数a1=1,排列321的逆序数a2=3,排列4321的逆序数a3=6．
Topic (Hunan, 2006, 20) In the arrangement p1p2...pm of m(m≥2) different numbers, if 1≤ipj (that is, some number is larger than the latter number). It is said that pi and pj constitute a reverse order. The total number of all reversed orders in an arrangement is called the inverse number of the permutations. It is noted that the inverse number of (n+1)n(n-1)...321 is an, such as the reverse order number a1=1 of the arrangement 21, the reverse order number a2=3 of the arrangement 321, and the reverse order number a3=6 of the arrangement 4321.