  We investigate the structure of power-closed ideals of the complex polynomial
ring $R = \mathbb{C}[x_1,\ldots,x_d]$ and the Laurent polynomial ring $R^{\pm}
= \mathbb{C}[x_1,\ldots,x_d]^{\pm} = M^{-1}\mathbb{C}[x_1,\ldots,x_d]$, where
$M$ is the multiplicative sub-monoid $M = [x_1,\ldots,x_d]$ of $R$. Here, an
ideal $I$ is {\em power-closed} if $f(x_1,\ldots,x_d)\in I$ implies
$f(x_1^i,\ldots,x_d^i)\in I$ for each natural $i$. In particular, we
investigate related closure and interior operators on the set of ideals of $R$
and $R^{\pm}$. Finally, we give a complete description of principal
power-closed ideals and of the radicals of general power-closed ideals of $R$
and $R^{\pm}$.
