A composite number is a positive integer which is not prime (i.e., which has factors other than 1 and itself). The first few composite numbers (sometimes called "composites" for short) are 4, 6, 8, 9, 10, 12, 14, 15, 16, ... (Sloane's A002808), whose prime decompositions are summarized in the following table. Note that the number 1 is a special case which is considered to be neither composite nor prime.

	prime factorization		prime factorization
4		20
6		21
8		22
9		24
10		25
12		26
14		27
15		28
16		30
18		32

The th composite number can be generated using the Mathematica code

  Composite[n_Integer] :=
    FixedPoint[n + PrimePi[#] + 1&, n]

There are an infinite number of composite numbers.

The composite number problem asks if there exist positive integers and such that .

A composite number can always be written as a product in at least two ways (since is always possible). Call these two products

(1)

then it is obviously the case that ( divides ). Set

(2)

where is the part of which divides , and is the part of which divides . Then there are and such that

			(3)
			(4)

Solving for gives

(5)

It then follows that

			(6)
			(7)
			(8)

It therefore follows that is never prime! In fact, the more general result that

(9)

is never prime for an integer also holds (Honsberger 1991).

Honsberger, R. More Mathematical Morsels. Washington, DC: Math. Assoc. Amer., pp. 19-20, 1991.

Sloane, N. J. A. Sequence A002808/M3272 in "The On-Line Encyclopedia of Integer Sequences."

CITE THIS AS:

Weisstein, Eric W. "Composite Number." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/CompositeNumber.html