This is an insight from lesson [AR1] Arithmetic, Number Theory
Number theory is a branch of mathematics that is devoted to studying natural numbers. These are numbers that are most familiar to you, starting with {1,2,3,...}, and reaching until infinity. As you can imagine, these numbers are quite fundamental for all of mathematics. Therefore Number Theory is also called the queen of mathematics.
If you wonder who the king of mathematics is, you have heard of him; Leonhard Euler. He created a mathematical formula that is all over the place in mathematics, physics and engineering. Look it up if you're interested.
When you look at all types of numbers in the illustration above, you see that next to natural numbers, there are whole numbers. This set of numbers is much the same as the set of natural numbers, however zero is included {0,1,2,...}. Take it another step broader and you will find integers, which are whole numbers that are either positive, negative, or exactly zero. Integer is a word that you will read a lot during your GMAT practice and during the exam. Examples of integers are the numbers 2 and 3, but √25 is also an integer, as you can express it as a number, in this case the number 5. When decimals are included, we speak of rational numbers.
Less common forms of numbers are termed irrational numbers, which are numbers that cannot be represented as a integer or decimal/fraction. A well-known example is 'Pi', which is used to circumscribe a circle and has a value of 3.14(...). The digits after the decimal point extend infinitely without a pattern, so strictly it is not possible to express Pi as a decimal, hence it is an irrational number.
Now do you want to dive deeper into the concept of number theory and understand these quotes?
All prime numbers except 2 and 5 end with either the digits 1, 3, 7 or 9.
When you encounter parentheses next to each other, it means you have to multiply them.
Then check out the preview of the course here.
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