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**Problem 13:**
"Show that the set \( E = \{2k : k \in \mathbb{N}\} \) is countably infinite."

### Solution Outline:

To show that the set \( E = \{2k : k \in \mathbb{N}\} \) is countably infinite, we need to demonstrate two key points:

1. **The set \( E \) is infinite**: 
   This requires us to show that there is no bound on the size of the set, i.e., it has infinitely many elements.

2. **The set \( E \) is countable**: 
   This means that there exists a one-to-one correspondence (a bijection) between the set \( E \) and the set of natural numbers \( \mathbb{N} \).

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### Step 1: Showing that \( E \) is infinite
The set \( E \) consists of even natural numbers, because each element of \( E \) is of the form \( 2k \) where \( k \in \mathbb{N} \). As \( k \) increases without bound (since \( \mathbb{N} \) is infinite), the values \( 2k \) also increase without bound. Therefore, the set \( E \) contains an infinite number of elements, since there is no upper limit on the values of \( k \) in \( \mathbb{N} \).

### Step 2: Showing that \( E \) is countable
To show that \( E \) is countable, we need to construct a bijection between \( \mathbb{N} \) (the set of natural numbers) and \( E \) (the set of even natural numbers). Define a function \( f : \mathbb{N} \to E \) by the rule:

\[
f(k) = 2k \quad \text{for all} \quad k \in \mathbb{N}.
\]

- This function is **injective** (one-to-one) because if \( f(k_1) = f(k_2) \), then \( 2k_1 = 2k_2 \), which implies \( k_1 = k_2 \).
  
- The function is also **surjective** (onto) because for every element \( e \in E \), we can find a corresponding \( k \in \mathbb{N} \) such that \( e = 2k \). Specifically, for any \( e \in E \), \( e \) must be of the form \( 2k \) for some \( k \in \mathbb{N} \), so \( f(k) = e \).

Since the function \( f \) is both injective and surjective, it is a bijection between \( \mathbb{N} \) and \( E \).

### Conclusion:
Since there exists a bijection between \( \mathbb{N} \) and \( E \), the set \( E = \{2k : k \in \mathbb{N}\} \) is countably infinite.

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Would you like further clarification on any part of this explanation?

Here are five related questions that can extend your understanding:

1. How would you prove that the set of odd numbers is countably infinite?
2. What is the definition of a countably infinite set in mathematical terms?
3. Can you describe another example of a set that is countably infinite?
4. What is the difference between countably infinite and uncountably infinite sets?
5. How can we extend this concept to show that all subsets of natural numbers are countable?

**Tip:** When proving a set is countable, finding a bijection with \( \mathbb{N} \) is a common strategy to establish the countability.

Show that the set E = {2k : k ∈ N} is countably infinite.

This problem demonstrates how to prove that the set of even natural numbers, E = {2k : k ∈ N}, is countably infinite by constructing a bijection between the natural numbers and the set E. Suitable for undergraduate students studying set theory.

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