# Prince Rupert's Cube

## Prince Rupert's Cube

Let $C$ be a unit cube.

The largest square tunnel that can be made in $C$ through which a larger cube may be passed has sides of length:

$\dfrac {3 \sqrt 2} 4 = 1 \cdotp 06066 \, 0$

## Source of Name

This entry was named for Prince Rupert of the Rhine.

## Historical Note

According to John Wallis, the puzzle now known as Prince Rupert's Cube was first posed by Prince Rupert of the Rhine in $1693$.

The correct answer was determined by Pieter Nieuwland.

An incorrect solution to this puzzle, often quoted in the literature, was provided by Wallis himself, who assumed that the tunnel in question would be parallel to the space diagonal of the cube.

This provides a solution of $\sqrt 6 - \sqrt 2 \approx 1 \cdotp 03527$.