Radical of an integer

In number theory, the radical of a positive integer n is defined as the product of the distinct prime numbers dividing n (each prime factor of n occurs exactly once as a factor of the product mentioned):

\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 multiplicative (but not completely multiplicative).

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.^[1] The definition is generalized to the largest t-free divisor of n, ${\mathrm {rad}}_{t}$ , which are multiplicative functions which act on prime powers as

{\mathrm {rad}}_{t}(p^{e})=p^{{{\mathrm {min}}(e,t-1)}}

The cases t=3 and t=4 are tabulated in A007948 and A058035.

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

↑ (sequence A007947 in the OEIS)

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

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Radical of an integer

Examples

Properties

See also

References