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The Positive Integers are Well Ordered

Most likely, you answered the questions posed in with an enthusiastic yes, in part because you wanted the shot at the money, but more concretely because it seems so natural.

The Positive Integers are Well Ordered

Most likely, you answered the questions posed in with an enthusiastic yes, in part because you wanted the shot at the money, but more concretely because it seems so natural. But you may be surprised to learn that this is really a much more complex subject than you might think at first. In , we discuss the development of the number systems starting from the Peano Postulates. Although we will not devote much space in this chapter to this topic, it is important to know that the positive integers come with some assembly required. In particular, the basic operations of addition and multiplication don't come for free; instead they have to be defined.

As a by-product of this development, we get the following fundamentally important property of the set \(\posints\) of positive integers:

An immediate consequence of the well ordered property is that the professor will indeed have to pay someone a dollareven if there are infinitely many students in the class.

Symbols used here

n!
factorial
n × (n−1) × … × 1; the number of orderings of n things. 0! = 1.
\binom{n}{k}
binomial coefficient, "n choose k"
Number of k-element subsets of n things: n!/(k!(n−k)!).
\sum_{k=1}^{n} a_k
summation
Add a_k for k = 1 up to n.
\prod_{k=1}^{n} a_k
product
Multiply a_k for k = 1 up to n.
\emptyset,\ |A|
empty set, cardinality
The set with no elements; the number of elements of A.

Questions people ask

Permutation or combination?

Ask whether order matters. A lock code is a permutation (order matters); a hand of cards is a combination (it does not).

What is a graph in this sense?

Dots (vertices) joined by lines (edges) — not a plot. Road maps, social networks and molecules are graphs; questions like "is there a route" and "how few colours" are graph theory.

Try your own

Parts of this page are adapted from Keller & Trotter, Applied Combinatorics (CC BY-SA 4.0). Condensed and re-explained here; errors are ours.

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