For which numbers $ n$ is there a positive integer $ k$ with the following property: The sum of digits for $ k$ is $ n$ and the number $ k^2$ has sum of digits $ n^2.$
To solve the problem, we need to find all numbers \( n \) for which there exists a positive integer \( k \) such that the sum of the digits of \( k \) is \( n \) and the sum of the digits of \( k^2 \) is \( n^2 \).

1. **Initial Observations**:
   - The sum of the digits of \( k \) is \( n \).
   - The sum of the digits of \( k^2 \) is \( n^2 \).

2. **Digit Constraints**:
   - If \( k \) satisfies the given condition, then \( 10^s k \) (where \( s \) is any non-negative integer) will also satisfy the condition because multiplying by \( 10^s \) only appends zeros to \( k \), which does not change the sum of the digits.
   - Therefore, we can assume \( 10 \nmid k \) (i.e., \( k \) does not end in zero).

3. **Induction on the Number of Digits of \( k \)**:
   - Let \( l \) be the number of digits of \( k \).
   - **Base Case**: \( l = 1 \)
     - If \( l = 1 \), then \( k \) can be \( 1, 2, \) or \( 3 \).
     - For \( k = 1 \), \( k^2 = 1 \), sum of digits of \( k^2 = 1 = 1^2 \).
     - For \( k = 2 \), \( k^2 = 4 \), sum of digits of \( k^2 = 4 = 2^2 \).
     - For \( k = 3 \), \( k^2 = 9 \), sum of digits of \( k^2 = 9 = 3^2 \).
   - **Inductive Step**: Assume the statement is true for \( l \) digits. We need to show it is true for \( l+1 \) digits.
     - Consider \( k \) with \( l+1 \) digits. Write \( k \) as \( 10^l p + q \) where \( p \) is a single digit and \( q \) has \( l \) digits.
     - We need to check the sum of the digits of \( k^2 = (10^l p + q)^2 = 10^{2l} p^2 + 2 \cdot 10^l p q + q^2 \).

4. **Case Analysis**:
   - **Case \( p = 3 \)**:
     - \( k = 100p + q \) where \( p = 3 \) and \( q \) has \( l \) digits.
     - \( k^2 = 90000 + 600q + q^2 \).
     - The sum of the digits of \( k^2 \) must be \( n^2 \).
     - This implies \( q \leq 1 \) (since higher values of \( q \) would make the sum of digits exceed \( n^2 \)).
     - Possible values of \( q \) are \( 1, 11 \).
   - **Case \( p = 2 \)**:
     - \( k = 100p + q \) where \( p = 2 \) and \( q \) has \( l \) digits.
     - \( k^2 = 40000 + 400q + q^2 \).
     - Possible values of \( q \) are \( 1, 2, 11, 12, 21 \).
   - **Case \( p = 1 \)**:
     - \( k = 100p + q \) where \( p = 1 \) and \( q \) has \( l \) digits.
     - \( k^2 = 10000 + 200q + q^2 \).
     - Possible values of \( q \) are \( 1, 2, 3, 11, 12, 13, 21, 22 \).

5. **Conclusion**:
   - By induction, we can see that for any \( n \), there exists a \( k \) such that the sum of the digits of \( k \) is \( n \) and the sum of the digits of \( k^2 \) is \( n^2 \).

The final answer is \( \boxed{ n } \) for any positive integer \( n \).