# THINGS YOU MAY NOT KNOW ABOUT NUMBERS

Senior Associate Editor Steven Cherry looks into a Web site that has something nice to say about nearly every integer from 0 to 9999.

Steven Cherry

Now this is what makes the Web special.

Erich Friedman, an Associate Professor of Mathematics at Stetson University, in DeLand, Florida, has a page called "What's Special About This Number?" It has a list of numbers from 0 to 9999 and states a unique property for each one.

Here are a few examples:

5 is the number of Platonic solids.

6 is the smallest perfect number.

7 is the smallest number of faces of a regular polygon that is not constructible by straightedge and compass.

8 is the largest cube in the Fibonacci sequence.

Don't know what a "perfect" number is? The site links to other pages around the Web that give detailed explanations. Mathematicians have tons of these technical terms (many of which use ordinary words, like "perfect").

I had a major in math, and while I knew what a perfect number was, there are plenty of terms and ideas here that I didn't learn in college, such as "hexomino."

Friedman has an unfortunate tendency to link to Wolfram's excellent MathWorld site, which gives extremely complicated, mathematically rigorous explanations ("perfect numbers are positive integers n such that n==s(n), where s(n) is the restricted divisor function (i.e., the sum of proper divisors of n), or equivalently sigma(n)==2n, where sigma(n) is the divisor function (i.e., the sum of divisors of n including n itself).")

But the Web is rife with simpler ones, and Google knows where they are. For practical purposes, a perfect number is any whose factors, not counting itself, add up to itself: 1+2+3=6. 1, 2, 4, 7, and 14 add up to 28, which is the second perfect number.

Anyway, Friedman's page is delightful. He doesn't find an interesting fact for every number between 0 and 9999, but he does for 2849 of them, which is remarkable. For example, 35 is the number of hexominoes, which are structures made up of six squares. The Wolfram explanation is more helpful here and it has a diagram:

Did you know that 38 is the last Roman numeral when written lexicographically? Or that 40 is the only number whose letters are in alphabetical order? That 727 has the property that its square is the concatenation of two consecutive numbers? I sure didn't.

If you know a distinctive fact about a number not listed here, please e-mail me.

We have a suggestion for the number 2600: It's the number of nuclear power plants needed to equal the energy contained in 1 cubic mile of oil, which happens to be amount the world currently uses annually. And 200 is the number of Three Gorges Dams it would take, and 5200 the number of coal-fired plants. You see, Spectrum's editors are almost as obsessed with numbers as Friedman, as they showed in an article last month, "Joules, BTUs, Quads-Let's Call the Whole Thing Off.")

Even without our energy numbers, over time, each of Friedman's 7151 missing factoids will be filled in, some of the existing ones will be improved, and the list will expand into the five-digit numbers and beyond.

The Web can be the sum of all knowledge, assembled collaboratively, lovingly, in a way that's never been possible before. We've put together a remarkable fraction of the world's knowledge so far. Imagine what it will look like in 100 years (the smallest square which is also the sum of 4 consecutive cubes), if we don't get in its way.