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Relations & Functions · Question 113 of 125

Question 113

MediumOriginal question

How many bijections f:{1,2,3,4,5}→{1,2,3,4,5}f : \{1, 2, 3, 4, 5\} \to \{1, 2, 3, 4, 5\} have f(1)≠1f(1) \ne 1?

Options for question 113

Show the answer and explanation
CorrectNot quiteAnswer B

Correct answer: B

Explanation

  1. There are 5!=1205! = 120 bijections; those with f(1)=1f(1) = 1 arrange the other 4 elements: 4!=244! = 24.

  2. 120−24=96120 - 24 = 96; or 4 choices for f(1)f(1), then 4!=244! = 24 for the rest: 4×24=964 \times 24 = 96.

Worth noting

119 removes only the identity map; every bijection with f(1)=1f(1) = 1, 24 of them, has to go.

Concept
Counting functions with a condition
Typical time
90 s
Your time
—
The rule this testsOnto and bijective functions
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Ten questions from Relations & Functions, at your own pace.

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Ten questions from Relations & Functions, at your own pace.