= Composite number
{wiki}
= Composite
{synonym}
A <composite number> is an <integer> greater than one that is not a <prime number>. Equivalently, it has a factorization $n=ab$ with <integers> $a,b>1$. Thus $4=2\cdot2$, $9=3\cdot3$, and $12=3\cdot4$ are <composite numbers>; one is neither a <composite number> nor a <prime number>.
Every <composite number> has a <prime factor> at most its <square root>. In $91=7\cdot13$, seven is the smaller factor and lies below $\sqrt{91}$; in $49=7^2$, equality occurs. This square-root bound makes <trial division> finite and lets the <sieve of Eratosthenes> stop marking with <prime numbers> beyond $\sqrt N$ when listing <prime numbers> up to $N$.
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