Let S := a + √2 b + √2 : a, b EN Prove that S is countably infinite.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Let S =
a + √2
b+ √2
: a, b EN. Prove that S is countably infinite.
NJ.
Transcribed Image Text:Let S = a + √2 b+ √2 : a, b EN. Prove that S is countably infinite. NJ.
Expert Solution
Step 1: Establishing a bijection

To prove that the set S defined as S={a+2b+2|a,bN} is countably infinite, need to establish a bijection between S and the set of natural numbers N.

One way to approach this is to notice that we can represent a+2b+2 in a form that will make it easier to work with. 

Let's multiplying both the numerator and denominator by the conjugate of the denominator:

S={(a+2)(b2)(b+2)(b2)|a,bN}

This simplifies to:

S={(a+2)(b2)(b22)|a,bN}

Now, it can see that every element in S can be represented as xy where x=(a+2)(b2) and y=b22.

Since a and b are natural numbers, both x and y are also natural numbers.


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