Find all pairs $(a,b)$ of real numbers such that $\lfloor an + b \rfloor$ is a perfect square, for all positive integer $n$.
1. **Assume \( a \neq 0 \)**:
   - If \( a \neq 0 \), then the expression \( \lfloor an + b \rfloor \) must be a perfect square for all positive integers \( n \).
   - Let \( \lfloor an + b \rfloor = k_n^2 \) for some integer \( k_n \) and for all \( n \in \mathbb{Z}^+ \).
   - Since \( \lfloor an + b \rfloor \) is a perfect square, the difference between consecutive perfect squares \( k_{n+1}^2 - k_n^2 \) must be at least 1.
   - However, the difference between \( \lfloor a(n+1) + b \rfloor \) and \( \lfloor an + b \rfloor \) is at most \( \lceil a \rceil \), which is a constant.
   - This implies that the sequence of perfect squares \( k_n^2 \) must be separated by intervals of at most \( \lceil a \rceil \), which is impossible because the difference between consecutive perfect squares grows without bound.
   - Therefore, \( a \) must be 0.

2. **Assume \( a = 0 \)**:
   - If \( a = 0 \), then the expression simplifies to \( \lfloor b \rfloor \).
   - For \( \lfloor b \rfloor \) to be a perfect square for all positive integers \( n \), \( b \) must be such that \( \lfloor b \rfloor \) is a constant perfect square.
   - Let \( \lfloor b \rfloor = k^2 \) for some non-negative integer \( k \).
   - This implies \( k^2 \leq b < k^2 + 1 \).

3. **Conclusion**:
   - The pairs \( (a, b) \) that satisfy the condition are \( (0, k^2 + u) \) where \( k \in \mathbb{Z}_{\ge 0} \) and \( u \in [0, 1) \).

The final answer is \( \boxed{ (0, k^2 + u) } \) where \( k \in \mathbb{Z}_{\ge 0} \) and \( u \in [0, 1) \).