 We now give a very quick tour of infinity in modern mathematics.  Section 3.1  reminds the reader of several familiar number systems: the natural numbers, integers, rational numbers, and real numbers.  Section 3.2  discusses the infinite operations of limits and sums that underlie calculus, and introduces the “extended real numbers” \(+\infty\) and \(-\infty\). The material in these two sections is covered in most textbooks on real analysis, and even many calculus textbooks, so some readers might already be familiar with it, while others might benefit from having such a textbook on hand to expand on some of the points.