Fix a nonnegative integer $a_0$ to define a sequence of integers $a_0,a_1,\ldots$ by letting $a_k,k\geq 1$ be the smallest integer (strictly) greater than $a_{k-1}$ making $a_{k-1}+a_k{}$ into a perfect square. Let $S{}$ be the set of positive integers not expressible as the difference of two terms of the sequence $(a_k)_{k\geq 0}.$ Prove that $S$ is finite and determine its size in terms of $a_0.$
1. **Define the sequence and initial conditions:**
   Let \( a_0 \) be a fixed nonnegative integer. Define the sequence \( a_0, a_1, a_2, \ldots \) such that for \( k \geq 1 \), \( a_k \) is the smallest integer strictly greater than \( a_{k-1} \) making \( a_{k-1} + a_k \) a perfect square. 

2. **Express the relationship in terms of perfect squares:**
   Suppose \( a_{k-1} + a_k = b_k^2 \) for some integer \( b_k \). Since \( a_k \) is the smallest integer greater than \( a_{k-1} \) making \( a_{k-1} + a_k \) a perfect square, we have:
   \[
   a_k = b_k^2 - a_{k-1}
   \]
   where \( b_k \) is the smallest integer such that \( b_k^2 > a_{k-1} \).

3. **Prove that \( b_{k+1} = b_k + 1 \):**
   We need to show that \( b_{k+1} = b_k + 1 \). Assume \( b_k \) is the smallest integer such that \( b_k^2 > a_{k-1} \). Then:
   \[
   a_k = b_k^2 - a_{k-1}
   \]
   For \( a_{k+1} \), we need \( a_k + a_{k+1} = b_{k+1}^2 \). Since \( a_{k+1} \) is the smallest integer greater than \( a_k \) making \( a_k + a_{k+1} \) a perfect square, we have:
   \[
   a_{k+1} = b_{k+1}^2 - a_k
   \]
   To ensure \( a_{k+1} \) is the smallest integer greater than \( a_k \), \( b_{k+1} \) must be the smallest integer such that \( b_{k+1}^2 > a_k \). Given \( a_k = b_k^2 - a_{k-1} \), we have:
   \[
   b_{k+1}^2 > b_k^2 - a_{k-1}
   \]
   Since \( b_{k+1} \) is the smallest integer greater than \( b_k \), it follows that \( b_{k+1} = b_k + 1 \).

4. **Determine the form of the differences:**
   Given \( b_{k+1} = b_k + 1 \), we can write:
   \[
   b_k = b_0 + k
   \]
   where \( b_0 \) is the initial value such that \( b_0^2 > a_0 \). The sequence \( a_k \) can be expressed as:
   \[
   a_k = (b_0 + k)^2 - a_{k-1}
   \]
   The differences \( a_{k+1} - a_k \) are given by:
   \[
   a_{k+1} - a_k = (b_0 + k + 1)^2 - (b_0 + k)^2 - a_k + a_{k-1}
   \]
   Simplifying, we get:
   \[
   a_{k+1} - a_k = 2(b_0 + k) + 1
   \]

5. **Determine the set \( S \):**
   The differences \( a_{k+1} - a_k \) form an arithmetic sequence with a common difference of 2. Therefore, all large enough odd numbers can be expressed as differences of two terms of the sequence. Similarly, all large enough even numbers can be expressed as differences of two terms of the sequence by considering the sum of two odd differences.

6. **Conclusion:**
   Since all large enough odd and even numbers can be expressed as differences of two terms of the sequence, the set \( S \) of positive integers not expressible as the difference of two terms of the sequence is finite. The size of \( S \) depends on the initial value \( a_0 \).

The final answer is \( \boxed{ S } \) is finite.