A countably infinite set
Is as simple as things like this get.
Just start counting at 1,
Then continue—it's fun!
I'll check back when you're done, so don't sweat.
Chris Doyle, The Omnificent English Dictionary in Limerick Form
In this chapter we will consider some basic concepts of the relations that are called functions. A large variety of mathematical ideas and applications can be more completely understood when expressed through the function concept.