## 11/11/11 11:11:11—The Right Time to Look at a Number

November 10, 2011 — Sol Lederman, Technical Communication & Strategy

World War I officially ended in 1918 at the 11th hour of the 11th day of the 11th month. Remembrance Day is observed in Commonwealth countries to recall the end of the war and to remember the members of the armed forces who gave their lives for the cause. This year we observe Remembrance Day on 11/11/11.

Beyond the somberness of this memorial day, those of us who are mathematically inclined consider the surprising ways we can combine 1s to achieve beautiful results. Some of these combinations involve rather unpleasant calculations, so we’ll let *Mathematica* do the heavy lifting while we marvel at the results!

Today I’ll share 11 interesting places in which 1 appears. Let’s jump in.

1. Can anything of interest come from combining the humble digit 1 with the square root and plus symbol? As the demonstration below suggests, given the right nesting, you get an infinite series that converges to *φ* = 1.61803…, the golden ratio.

2. Interestingly enough, a different combination of an infinite series of 1s also produces a series that converges to the golden ratio.

3. I would be remiss if I didn’t find a way to include *π* in this digits game. Here’s a series that (slowly) converges to *π*. Thank goodness that we don’t have to compute these products by hand!

4. Something interesting happens when we compute powers of 11. In particular, the digits of the first five powers of 11, through 11^{4}, correspond to the entries of the first five rows of Pascal’s triangle. (Can you see why this correspondence exists for these few terms and why it doesn’t exist beyond 11^{4}?)

5. What do you get when you multiply two numbers each consisting of all 1s? You get a pretty product.

6. Have you ever wondered what *π*(*π*^{π}) might be? In this expression, *π* is the number theory function (`PrimePi` in *Mathematica*) that tells how many primes there are that are less than or equal to a given number.

7. How quickly can you compute the last 11 digits of 11^{11(11×11)}? This number is so huge that it has more digits than the number of particles in the universe. Fortunately, *Mathematica* can quickly compute those last digits.

8. Did you know that if a number is divisible by 11, then the number you get when you reverse its digits is also divisible by 11? Below is an example. (Note that `FactorInteger`[*n*] gives a list of the prime factors of the integer *n* together with their exponents.)

9. What’s special about 11!+11!+11!+11!+11!+1? Yes, it’s prime.

10. Here’s a fun factoid that’s not at all related to *Mathematica*, but nonetheless worth including in any discussion about the number 11. Guess how many sides the polygon has that surrounds the portrait of Susan B. Anthony on the U.S. dollar coin? After you’ve answered that question, here’s a harder one. What do you call an 11-sided polygon? Hint: Wolfram|Alpha can help.

11. Our final look at the digit 1 is an exploration. How many diagonals are there in a regular 11-sided polygon? How could you figure this out with and without *Mathematica*?

I would like to thank the readers of *Wild About Math!* and the creators of the prime pages site for the suggestions, many of which inspired this article.

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## 5 Comments

Amazing!!! Just with ones you can create all the other numbers, including 0: 1-1=0!

In 5., a challenge, try with n=9!

Very interesting. If your expression 11^11^(11*11) has more particles than in the universe, what expression would represent the number of particles in the universe?

Brilliant

This is a _el_-ev__en___, _el_-_eve__n_, _elev_e__n_ _el___e _ven_ ___ _e_le___ven___. (A well-evidenced, self-reverent, televiewing welcome event for reflectiveness.)

[...] I begin, the WolframAlpha blog has 11 interesting ways the number 1 can be used to create some interesting mathematical results. I now know, for example, that an 11-sided polygon is called a [...]