What is the probability that only 2 colors are chosen?

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Discussion Overview

The discussion revolves around calculating the probability that only 2 colors are chosen when n people randomly select from C available colors. The scope includes mathematical reasoning and combinatorial analysis.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant introduces the problem and seeks to determine the probability of exactly 2 colors being chosen.
  • Another participant suggests a method to calculate this probability by first selecting a pair of colors and then counting the onto functions from a set of size n to a set of size 2, proposing the use of bit strings to represent the functions.
  • A later post indicates that the initial poster has resolved the problem independently.

Areas of Agreement / Disagreement

The discussion does not reach a consensus, as the initial poster claims to have solved the problem without sharing the solution, leaving the method and final probability unresolved.

Contextual Notes

Limitations include the lack of detailed steps in the proposed method for counting onto functions and the absence of the final solution or verification of the initial poster's claim of resolution.

bob j
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Hi all,
I have the following problem. n people choose a color at random. There are C available colors, from 1 to C, for each node.

What is the probability that only 2 colors are chosen?

thanks
 
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Well, to find the number of ways for the total number of colors chosen to be exactly 2, first pick the pair of colors, which can be done in C(C,2) ways, with C(,) being choose. Then count the number of onto functions from a set of size n to a set of size 2.

You could find the latter as follows: every function from a set of size n to a set of size 2, can be represented as a bit string of length n, containing at least one 0 and at least one 1. How many such bit strings are there?
 


never mind, I solved it ;)
 


Great
 

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