w***d
发帖数: 17 | 1 let f(n,r) be the number of ways to arrange the numbers 1,2, ..., n in a line
so that exactly r numbers match their correct positions. For example, if n =5,
then 1 4 3 5 2 has 2 matches: 1 is in the 1st position, and 3 is in the 3rd
positions; 1 2 3 4 5 has 5 matches.
Then is there an explicit expression for f(n,r)? Thx a lot!!! | w***d
发帖数: 17 | 2 哪位大侠帮个忙啊，着急要阿
line
5,
【在 w***d 的大作中提到】
: let f(n,r) be the number of ways to arrange the numbers 1,2, ..., n in a line
: so that exactly r numbers match their correct positions. For example, if n =5,
: then 1 4 3 5 2 has 2 matches: 1 is in the 1st position, and 3 is in the 3rd
: positions; 1 2 3 4 5 has 5 matches.
: Then is there an explicit expression for f(n,r)? Thx a lot!!!
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