A positive integer number is written in red on each side of a square. The product of the two red numbers on the adjacent sides is written in green for each corner point. The sum of the green numbers is $40$. Which values ​​are possible for the sum of the red numbers?

(G. Kirchner, University of Innsbruck)
1. Let \( a, b, c, d \) be the four red numbers written on the sides of the square. The green numbers at the corners are the products of the red numbers on the adjacent sides. Therefore, the green numbers are \( ab, bc, cd, \) and \( da \).

2. According to the problem, the sum of the green numbers is given as:
   \[
   ab + bc + cd + da = 40
   \]

3. We can rearrange the terms to group them by pairs:
   \[
   ab + da + bc + cd = 40
   \]
   Notice that this can be factored as:
   \[
   a(b + d) + c(b + d) = 40
   \]
   This simplifies to:
   \[
   (a + c)(b + d) = 40
   \]

4. Let \( x = a + c \) and \( y = b + d \). Then we have:
   \[
   xy = 40
   \]

5. Since \( x \) and \( y \) are sums of positive integers, they must be positive integers themselves. We need to find pairs \((x, y)\) such that their product is 40. The possible pairs \((x, y)\) are:
   \[
   (1, 40), (2, 20), (4, 10), (5, 8), (8, 5), (10, 4), (20, 2), (40, 1)
   \]

6. The sum of the red numbers \( a + b + c + d \) is given by:
   \[
   a + b + c + d = (a + c) + (b + d) = x + y
   \]

7. For each pair \((x, y)\), we calculate \( x + y \):
   \[
   \begin{align*}
   (1, 40) & : 1 + 40 = 41 \\
   (2, 20) & : 2 + 20 = 22 \\
   (4, 10) & : 4 + 10 = 14 \\
   (5, 8) & : 5 + 8 = 13 \\
   (8, 5) & : 8 + 5 = 13 \\
   (10, 4) & : 10 + 4 = 14 \\
   (20, 2) & : 20 + 2 = 22 \\
   (40, 1) & : 40 + 1 = 41 \\
   \end{align*}
   \]

8. The unique possible values for the sum of the red numbers \( a + b + c + d \) are:
   \[
   13, 14, 22, 41
   \]

The final answer is \(\boxed{13, 14, 22, 41}\)