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Let's review those definitions:
An edge can even join a vertex to itself, which is called a loop in a graph.
Each time we add a new point (vertex), we have to connect it to the other points (vertices): so how do the number of connections (arcs) grow with the number of vertices (points)? We came up with a formula:
\[ \text{arcs}(n\ \text{vertices}) = \frac{n(n-1)}{2} \]
We realized that this is the sum of the first \(n-1\) natural numbers
Let's revisit that, with \(K_5\):
I hope that you noticed the pretty geometric way of envisioning this in "Rock Groups", where the sum is the natural numbers from 1 to 10 (55):
I want to think back to the number 6, and beer:
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Numbers (like squares), which can be represented by some geometric figure are called polygonal numbers. Another interesting example given in Rock Groups, are the perfect squares: \(n^2\) can be thought of as the sum of the first \(n\) odd natural numbers:
\(1+3+5+7+9\) | \(=25\) |
These mysterious properties are parts of the personalities of numbers. What's your favorite natural number's personality? If your favorite number is 36, then you're lucky: it's both square and triangular!
It turns out that The Great Fraudini can read anyone's mind -- so long as they're thinking of a number between 1 and 63, and they're willing to play a little card game with him. So let's see how this works....
So in case you don't have a magic trick up your sleeve, I'm giving you one!
However I want you to think about it before I just give it away. Take a look at your cards, and see if you can figure out how The Great Fraudini is doing it....
But I also always tell my students that gambling is foolish, unless you have inside information. So I'm all about giving you the inside information. But generally, and (in particular):
But how might "primitive" people have counted? (Ernie's not primitive enough; neither is he a person.) You won't be surprised to learn that we will be using a tree to help us.
You make a note of whether there is a leftover sheep or not -- maybe you make a mark, like a "1" or a "0". This is all you have to do to communicate the number to the King!
But you must also pay attention to the direction in which you write the marks.
Let's see how we might use a tree to represent the solution to the "22" counting problem: we'll use a ternary tree (three children).
I say "Whoever's doing this..." -- I mean, "Whoever's doing this primitive counting...." This is presumably someone who doesn't know how to count -- at least not the way we do -- but they can tell if there's one left over (and write "1") or not (and write "0").
Notice that we eventually have a single sheep in a pen, and that's when we're done. We have to write a "1" for the final sheep, to indicate that there's "one leftover".
Then the answer will be written as 1, 0, 1, 1, 0. That is, from the bottom up, left to right. This is important! We have to have a consistent scheme for writing (or otherwise recording our results -- perhaps on a knotted string).
So how do we write
(Notice that, while there can be either a 0 or 1 at the right end, there is always the "1" on the left -- meaning the last sheep standing!
Can you rebuild the tree using these "tally sticks"? If so, you can get a job in the King's counting house.
But rather than count out fifty pennies, I'd make a stack of 10, then four more stacks of exactly the same height (one-to-one correspondence). Five stacks of 10 makes 50. This is a similar idea....
If you have a lot of pennies, you could divide them in half, count half, and then multiply by two. But if you have one left over, you'd have to add that one.
If the half is still too many to count, do it again (on half), and so on -- until you get to the point where you can count "the half" (like if it gets down to 1 coin, say!:).