The numbers 1, 2, 3, 6, 9, 18 form a sequence of positive integers. They are all factors of the number 18, meaning each number can divide 18 without leaving a remainder. This set includes both prime numbers, like 2 and 3, and composite numbers, such as 6 and 18. In this sequence, 1 is the smallest factor, while 18 is the largest. The numbers can also be grouped into pairs that multiply to 18: 1 and 18, 2 and 9, and 3 and 6. This illustrates the relationship between factors and their products.