Let's start! Suppose you have lots of left-over postage stamps¹Perhaps because you only use email or texting now; too bad for you! that are of just a few different denominations. It could be fun to see what amounts you could make from them.

To be concrete, let's assume first that all your stamps are numbered 2¢ and 3¢. Here are two questions we could ask. They are mathematically equivalent, but might take your exploration in two very different directions!

• Which denominations of postage can you get by combining just these kinds?

• Which denominations can you not get with just these two kinds?

Once you've thought about that, try the same problem with 2¢ and 4¢ stamps. What is the same, what is different?

Now let's get to a nontrivial case; what about with 3¢ and 4¢ stamps? In this case, after some experimentation, it looks like only 1, 2, and 5 are not possible, so anything six or above is possible. We call this number the conductor of the set \(\{3,4\}\).

What we are really asking, which might be clear by now, is which positive integers \(n\) are impossible (or possible) to write in the form \(n=3x+4y\), for some nonnegative integers \(x\) and \(y\). This is also sometimes called the Frobenius²For a very full discussion, see [C.6.20]. But not until after you have started the next chapter of this book! or coin problem.

Continue trying this with different small pairs of numbers (see also Exercise 1.4.5–Exercise 1.4.7). Pay attention to two things:

• What is the conductor of the pair? (You might want to ask whether there is such a number!)

• How many numbers lower than the conductor cannot be written in this way?