Home > Geometry, Technology > The Greatest Parallelepiped Ever

The Greatest Parallelepiped Ever


In general, it’s unusual for a rectangle to have sides and diagonals whose lengths are all integers (i.e., whole numbers).  Consider the following three rectangles, all of width 3:

Looking at the different lengths, we see one place where the diagonal length is an integer, but in the other cases, the diagonal length happens to be a non-terminating, non-repeating decimal (i.e., irrational).  Indeed, the diagonal length will be an integer exactly when the length and width are part of a Pythagorean triple, but compared to the alternative, this is uncommon.  (While there are infinitely many occurrences of this, we can still meaningfully consider it uncommon).

Now, imagine the situation in three dimensions.  A rectangular prism (think of a cardboard box) has 12 sides, 12 face diagonals, and four space diagonals.  It would be extremely unusual for all of those 28 lengths to be integers.  Even if we didn’t limit ourselves to rectangular prisms, but we allowed for the box to be slanted in all directions (that is, a parallelepiped), it would still be a numerical miracle for all those lengths to be integers.

Well, meet the perfect parallelepiped!

This was discovered by a couple of mathematicians at Lafayette College in Pennsylvania, using brute-force computer trials.  It looks like they found some others, too.   So thank you, Clifford Reiter and Jorge Sawyer, for giving me an extra credit problem for my next exam!

Click here to see more in Geometry.

www.MrHonner.com

  1. Ahmed Gouda
    October 24, 2010 at 1:13 am

    I thought about just making a bunch of equations, and seeing if it would be possible to find this Parallelepiped without a computer. Its proving more difficult then expected.

  2. October 24, 2010 at 7:58 am

    That’s a lot of vector algebra.

  3. DHC
    October 25, 2010 at 6:55 am

    Is it perfect only because it has integral lengths?

  4. October 25, 2010 at 5:48 pm

    Yes, that’s the definition of a perfect parallelepiped. Apparently the guys linked to above were the first ones to show that these things existed.

    This demonstration at WolframMathWorld is pretty cool:
    http://demonstrations.wolfram.com/PerfectParallelepipeds/

  1. May 2, 2011 at 9:10 am

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: