# 2: The Integers


Today, much as we take for granted the fact that there exists a number zero, denoted by 0, such that a+0 = a for any whole number a, we similarly take for granted that for any whole number a there exists a unique number −a, called the “negative” or “opposite” of a, so that a + (−a) = 0.

In a natural way, or so it seems to modern-day mathematicians, this easily introduces the concept of a negative number. However, history teaches us that the concept of negative numbers was not embraced wholeheartedly by mathematicians until somewhere around the 17th century.

In his work Arithmetica (c. 250 AD), the Greek mathematician Diophantus (c. 200-284 AD), who some call the “Father of Algebra,” described the equation 4 = 4x + 20 as “absurd,” for how could one talk about an answer less than nothing? Girolamo Cardano (1501-1576), in his seminal work Ars Magna (c. 1545 AD) referred to negative numbers as “numeri ficti,” while the German mathematician Michael Stifel (1487-1567) referred to them as “numeri absurdi.” John Napier (1550-1617) (the creator of logarithms) called negative numbers “defectivi,” and Rene Descartes (1596-1650) (the creator of analytic geometry) labeled negative solutions of algebraic equations as “false roots.”

On the other hand, there were mathematicians whose treatment of negative numbers resembled somewhat our modern notions of the properties held by negative numbers. The Indian mathematician Brahmagupta described arithmetical rules in terms of fortunes (positive number) and debts (negative numbers). Indeed, in his work Brahmasphutasiddhanta, he writes “a fortune subtracted from zero is a debt,” which in modern notation would resemble 0 − 4 = −4.

If you find the study of the integers somewhat difficult, do not be discouraged, as centuries of mathematicians have struggled mightily with the topic. With this thought it mind, let’s begin the study of the integers.

• 2.1: An Introduction to the Integers
Negative numbers have a rich and storied history. One of the earliest applications of negative numbers had to do with credits and debits. For example, if $5 represents a credit or profit, then −$5 represents a debit or loss. Note that if a vendor experiences a profit of $5 on a sale, then a loss of −$5 on a second sale, the vendor breaks even, i.e., the sum of $5 and −$5 is zero. In much the same way, every whole number has an opposite or negative counterpart.