Let $A_o =\{1, 2\}$ and for $n> 0, A_n$ results from $A_{n-1}$ by adding the natural numbers to $A_{n-1}$ which can be represented as the sum of two different numbers from $A_{n-1}$. Let  $a_n = |A_n |$ be the number of numbers in $A_n$. Determine $a_n$ as a function of  $n$.
1. **Base Case:**
   We start with \( A_0 = \{1, 2\} \). Clearly, \( |A_0| = 2 \).

2. **Inductive Hypothesis:**
   Assume that for some \( n \geq 0 \), \( A_n = \{1, 2, \ldots, m\} \) for some integer \( m \), and \( |A_n| = m \).

3. **Inductive Step:**
   We need to show that \( A_{n+1} \) is of the form \( \{1, 2, \ldots, 2m-1\} \) and that \( |A_{n+1}| = 2m - 1 \).

   - By the problem's construction, \( A_{n+1} \) is formed by adding to \( A_n \) all natural numbers that can be represented as the sum of two different numbers from \( A_n \).
   - Since \( A_n = \{1, 2, \ldots, m\} \), the smallest sum of two different elements is \( 1 + 2 = 3 \) and the largest sum is \( (m-1) + m = 2m - 1 \).
   - Therefore, \( A_{n+1} = A_n \cup \{m+1, m+2, \ldots, 2m-1\} \).

4. **Verification:**
   - We need to verify that \( A_{n+1} = \{1, 2, \ldots, 2m-1\} \).
   - Since \( A_n = \{1, 2, \ldots, m\} \), and we are adding all sums of two different elements from \( A_n \), we get all numbers from \( m+1 \) to \( 2m-1 \).
   - Thus, \( A_{n+1} = \{1, 2, \ldots, 2m-1\} \).

5. **Recurrence Relation:**
   - From the above, we see that \( |A_{n+1}| = 2m - 1 \).
   - Since \( |A_n| = m \), we have \( |A_{n+1}| = 2|A_n| - 1 \).

6. **Solving the Recurrence:**
   - We start with \( |A_0| = 2 \).
   - Using the recurrence relation \( |A_{n+1}| = 2|A_n| - 1 \), we can solve for \( |A_n| \).

   Let's denote \( a_n = |A_n| \):
   \[
   a_{n+1} = 2a_n - 1
   \]
   - For \( n = 0 \), \( a_0 = 2 \).
   - For \( n = 1 \), \( a_1 = 2a_0 - 1 = 2 \cdot 2 - 1 = 3 \).
   - For \( n = 2 \), \( a_2 = 2a_1 - 1 = 2 \cdot 3 - 1 = 5 \).
   - For \( n = 3 \), \( a_3 = 2a_2 - 1 = 2 \cdot 5 - 1 = 9 \).

   We observe that \( a_n = 2^n + 1 \).

   To prove this by induction:
   - **Base Case:** For \( n = 0 \), \( a_0 = 2 = 2^0 + 1 \).
   - **Inductive Step:** Assume \( a_k = 2^k + 1 \) for some \( k \geq 0 \). Then,
     \[
     a_{k+1} = 2a_k - 1 = 2(2^k + 1) - 1 = 2^{k+1} + 2 - 1 = 2^{k+1} + 1
     \]
   - By induction, \( a_n = 2^n + 1 \) for all \( n \geq 0 \).

The final answer is \( \boxed{ a_n = 2^n + 1 } \).