In mathematics, a squarefree, or quadratfrei, integer is one divisible by no perfect square, except 1. For example, 10 is squarefree but 18 is not, as it is divisible by 9 = 3^{2}. The smallest squarefree numbers are
Ring theory generalizes the concept of being squarefree.
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Equivalent characterizations of squarefree numbers
The positive integer n is squarefree if and only if in the prime factorization of n, no prime number occurs more than once. Another way of stating the same is that for every prime factor p of n, the prime p does not divide n / p. Yet another formulation: n is squarefree if and only if in every factorization n=ab, the factors a and b are coprime. An immediate result of this definition is that all prime numbers are squarefree.
The positive integer n is squarefree if and only if μ(n) ≠ 0, where μ denotes the Möbius function.
The Dirichlet series that generates the squarefree numbers is
This is easily seen from the Euler product
The positive integer n is squarefree if and only if all abelian groups of order n are isomorphic, which is the case if and only if all of them are cyclic. This follows from the classification of finitely generated abelian groups.
The integer n is squarefree if and only if the factor ring Z / nZ (see modular arithmetic) is a product of fields. This follows from the Chinese remainder theorem and the fact that a ring of the form Z / kZ is a field if and only if k is a prime.
For every positive integer n, the set of all positive divisors of n becomes a partially ordered set if we use divisibility as the order relation. This partially ordered set is always a distributive lattice. It is a Boolean algebra if and only if n is squarefree.
The radical of an integer is always squarefree.
Distribution of squarefree numbers
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