Greeting Card issue

Source: Internet
Author: User

Four people send greeting cards to each other, asking everyone not to get their own greeting cards. How many sending methods are there? N people?

Google is the answer to this question. We remember that N people send greeting cards to each other. Each person cannot receive his/her greeting card. Below are some answers:

The above is a recursive formula circulating on the internet, and I want to record a different recursive formula:

To prove that it is easy, you only need to know the generic formula rather than the recursive formula:

(Note: This recursive formula was obtained ten years ago in the Course of High School Mathematics Learning and is now updated to the blog)

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