 is called a "prime number" (or a "prime") if it is greater than 1 and cannot be written as the product of two smaller natural numbers.The numbers greater than 1 that are not prime are called composite numbers.In other words, formula_1 is prime if formula_1 items cannot be divided up into smaller equal-size groups of more than one item, or if it is not possible to arrange formula_1 dots into a rectangular grid that is more than one dot wide and more than one dot high.