Science:Math Exam Resources/Courses/MATH220/April 2005/Question 03 (a)
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Question 03 (a) 

Define what it means for two sets A and B to have the same cardinality. 
Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you? 
If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! 
Hint 

An example is not a definition. But it sure helps to have some examples and counterexamples to understand what you are trying to define. 
Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work.

Solution 

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Please rate my easiness! It's quick and helps everyone guide their studies. Two sets A and B are said to have the same cardinality if there exists a bijection from one set to the other. More precisely, that means there must exist at least one function ƒ: A → B such that the function is injective and surjective; that is, for each element a of the set A there exists a unique element b in the set B such that ƒ(a) = b (that condition is the injectivity) AND for each element b of the B there exists a unique element a in the set A such that ƒ(a) = b (and this is the condition for surjectivity). 