Mathematics for Computer Science

“mcs” — 2017/3/3 — 11:21 — page 304 — #312
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Chapter 9
Number Theory
9.4 The Fundamental Theorem of Arithmetic
There is an important fact about primes that you probably already know: every
positive integer number has a unique prime factorization. So every positive integer
can be built up from primes in exactly one way. These quirky prime numbers are
the building blocks for the integers.
Since the value of a product of numbers is the same if the numbers appear in a
different order, there usually isn’t a unique way to express a number as a product
of primes. For example, there are three ways to write 12 as a product of primes:
12 D 2 2 3 D 2 3 2 D 3 2 2:
What’s unique about the prime factorization of 12 is that any product of primes
equal to 12 will have exactly one 3 and two 2’s. This means that if we sort the
primes by size, then the product really will be unique.
Let’s state this more carefully. A sequence of numbers is weakly decreasing
when each number in the sequence is at least as big as the numbers after it. Note
that a sequence of just one number as well as a sequence of no numbers—the empty
sequence—is weakly decreasing by this definition.
Theorem 9.4.1. [Fundamental Theorem of Arithmetic] Every positive integer is a
product of a unique weakly decreasing sequence of primes.
For example, 75237393 is the product of the weakly decreasing sequence of
primes
23; 17; 17; 11; 7; 7; 7; 3;
and no other weakly decreasing sequence of primes will give 75237393. 8
Notice that the theorem would be false if 1 were considered a prime; for example,
15 could be written as 5 3, or 5 3 1, or 5 3 1 1, . . . .
There is a certain wonder in unique factorization, especially in view of the prime
number mysteries we’ve already mentioned. It’s a mistake to take it for granted,
even if you’ve known it since you were in a crib. In fact, unique factorization
actually fails for many integer-like sets of numbers, such as the complex numbers
of the form n C m p 5 for m; n 2 Z (see Problem 9.25).
The Fundamental Theorem is also called the Unique Factorization Theorem,
which is a more descriptive and less pretentious, name—but we really want to get
your attention to the importance and non-obviousness of unique factorization.
8 The “product” of just one number is defined to be that number, and the product of no numbers is
by convention defined to be 1. So each prime p is uniquely the product of the primes in the lengthone
sequence consisting solely of p, and 1, which you will remember is not a prime, is uniquely the
product of the empty sequence.