Show that no integer of the form $ xyxy$ in base $ 10$ can be a perfect cube. Find the smallest base $ b>1$ for which there is a perfect cube of the form $ xyxy$ in base $ b$.
1. **Show that no integer of the form $xyxy$ in base 10 can be a perfect cube.**

   An integer of the form $xyxy$ in base 10 can be written as:
   \[
   \overline{xyxy} = 1000x + 100y + 10x + y = 1010x + 101y = 101(10x + y)
   \]
   Assume that $\overline{xyxy}$ is a perfect cube. Then, since $101$ is a prime number, for $\overline{xyxy}$ to be a perfect cube, $101$ must divide $10x + y$. However, $10x + y$ is a two-digit number, and the maximum value it can take is $99$. Since $101 > 99$, $101$ cannot divide $10x + y$. Therefore, $\overline{xyxy}$ cannot be a perfect cube in base 10.

2. **Find the smallest base $b > 1$ for which there is a perfect cube of the form $xyxy$ in base $b$.**

   An integer of the form $xyxy$ in base $b$ can be written as:
   \[
   \overline{xyxy}_b = xb^3 + yb^2 + xb + y = (xb + y)(b^2 + 1)
   \]
   We need $(xb + y)(b^2 + 1)$ to be a perfect cube. Consider two cases:

   **Case 1:** $b^2 + 1$ is square-free (i.e., it has no repeated prime factors).

   For $(xb + y)(b^2 + 1)$ to be a cube, $(b^2 + 1)^2$ must divide $xb + y$. However, $xb + y < 2b^2 < (b^2 + 1)^2$, which is a contradiction. Therefore, $b^2 + 1$ must not be square-free.

   **Case 2:** $b^2 + 1$ is not square-free.

   We need to find the smallest $b$ such that $b^2 + 1$ is not square-free. Checking small values of $b$:
   - For $b = 2$: $b^2 + 1 = 5$ (square-free)
   - For $b = 3$: $b^2 + 1 = 10$ (square-free)
   - For $b = 4$: $b^2 + 1 = 17$ (square-free)
   - For $b = 5$: $b^2 + 1 = 26$ (square-free)
   - For $b = 6$: $b^2 + 1 = 37$ (square-free)
   - For $b = 7$: $b^2 + 1 = 50 = 2 \times 5^2$ (not square-free)

   Therefore, the smallest $b$ such that $b^2 + 1$ is not square-free is $b = 7$.

   When $b = 7$, we have:
   \[
   \overline{xyxy}_7 = (7x + y)(7^2 + 1) = (7x + y)(50)
   \]
   We need $(7x + y)(50)$ to be a perfect cube. Checking small values of $x$ and $y$, we find:
   \[
   (7 \times 2 + 6)(7^2 + 1) = (14 + 6)(50) = 20 \times 50 = 1000 = 10^3
   \]
   Therefore, $1000$ is a perfect cube, and the smallest base $b$ for which there is a perfect cube of the form $xyxy$ is $b = 7$.

The final answer is $\boxed{7}$.