# Common Multiples and the Least Common Multiple

The purpose of the applet below is to illustrate the notion of a *multiple* and the *least common multiple* of two integers. For an integer P, a *multiple* is any other integer divisible by P. Any integer P, has an infinite sequence of multiples. Speaking formally, multiples may be both negative and positive (and 0 of course.) For example, -10, -5, 0, 5, 10 are all multiples of 5 and of -5(!) as is any number in the form 5k, where k is any integer. However, most often we focus exclusively on positive integers and on their positive multiples.

The applet displays to the number line (x-axis) and two rows of red dots, above and below the line. Each of the rows is preceded by a hollow moveable dot. The hollow dots designate two integers, say, P and Q. At the beginning,

Some numbers appear in both rows and these are emphasized with blue dots on the line itself. These are the common multiples of P and Q, i.e., the numbers which are both a multiple of P and a multiple of Q. Since we are only concerned with positive integers, among all common multiples of two integers P and Q, there is the smallest. This is the leftmost of the blue dots. The number is called the *Least Common Multiple* of P and Q:

What if applet does not run? |

Both P and Q are factors of lcm(P, Q): P|lcm(P, Q) and also Q|lcm(P, Q). lcm(P, Q) is the least positive integer with this property. The least common multiple of two integers can be found with a factor tree or with the onion algorithm.

The applet may suggest the following

### Theorem 1

For two integers P and Q,

lcm(P, Q) = Q if and only if P|Q.

### Proof

P is a factor of lcm(P, Q), i.e. P|lcm(P, Q). If lcm(P, Q) = Q, then P|Q.

In the opposite direction, the fact that P|Q means that Q is a multiple of P; and, since Q is also a multiple of itself, Q is the common multiple of both P and Q. But Q is certainly the smallest positive multiple of itself, so that no common multiple of P and Q may be smaller than Q. Which shows that

The applet helps make another observation: all common multiples of two integers P and Q are divisible by (or are multiples of)

### Theorem 2

For three integers P, Q, R,

if P|R and Q|R then lcm(P, Q)|R.

In other words if R is divisible by both P and Q, i.e., if R is a common multiple of P and Q, then it is divisible by

### Proof

Assume, S is another common multiple of P and Q, i.e. P|S and Q|S. For simplicity, let

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Copyright © 1996-2018 Alexander Bogomolny