Radical of an integer

In number theory, the radical of a positive integer n is defined as the product of the prime numbers dividing n:

$\displaystyle \mathrm {rad} (n)=\prod _{\scriptstyle p\mid n \atop p{\text{ prime}}}p.\,$

Examples

Radical numbers for the first few positive integers are

1, 2, 3, 2, 5, 6, 7, 2, 3, 10, ... (sequence A007947 in the OEIS).

For example,

$504=2^{3}\cdot 3^{2}\cdot 7$

and therefore

$\mathrm {rad} (504)=2\cdot 3\cdot 7=42.\,$

Properties

The function $\mathrm {rad}$ is not multiplicative. Indeed, $2=\mathrm {rad} (4)=\mathrm {rad} (2\cdot 2)\neq \mathrm {rad} (2)\cdot \mathrm {rad} (2)=4$.

The radical of any integer n is the largest square-free divisor of n, and so also described as the square-free kernel of n.

One of the most striking applications of the notion of radical occurs in the abc conjecture, which states that, for any ε > 0, there exists a finite K_ε such that, for all triples of coprime positive integers a, b, and c satisfying a + b = c,

$c<K_{\varepsilon }\,\operatorname {rad} (abc)^{1+\varepsilon }.$

Furthermore, it can be shown that the nilpotent elements of $\mathbb {Z} /n\mathbb {Z}$ are all of the multiples of rad(n).

References

• Guy, Richard K. (2004). Unsolved Problems in Number Theory. Springer-Verlag. p. 102. ISBN 0-387-20860-7.