Number of functions from a to b where {123} is in the range of (f)

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SUMMARY

The discussion focuses on calculating the number of functions from set A, containing n elements, to set B = {0, 1, 2, 3}, ensuring that the range of the function includes the elements {1, 2, 3}. The total number of valid functions is derived using the formula n*(n-1)*(n-2) * 4^(n-3), accounting for the requirement that {1, 2, 3} must be included in the range. The inclusion-exclusion principle is employed to avoid double-counting functions that may have repeated elements.

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Homework Statement


A has n elements.
B={0,1,2,3}
{1,2,3}⊆range(f)

Homework Equations

The Attempt at a Solution


So in each function we must choose those 3 numbers in the range.
So let's first choose all the diffrent possiblites to choose those 3:
n*(n-1)*(n-2)

now for the remaining elemnts, we should have 4n-3

So totally we should have n*(n-1)*(n-2) * 4n-3 diffrent functions.
 
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You can't separate them like that. You would e.g. count (1,2,3,3) twice, once with the first 3 "chosen" and once with the second one.
You can use inclusion-exclusion: How many functions are there in total? How many without 3? How many without 2? Without 1? How many did you subtract twice now?
 

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