Geometry of integral polynomials, $M$-ideals and unique norm preserving extensions

We use the Aron-Berner extension to prove that the set of extreme points of the unit ball of the space of integral polynomials over a real Banach space $X$ is $\{\pm {\varphi }^k:\varphi \in X^{\ast },\mathrm{\parallel }\varphi \mathrm{\parallel }=1\}$. With this description we show that, for real Banach spaces $X$ and $Y$, if $X$ is a non trivial $M$-ideal in $Y$, then ${\widehat{⨂}}_{{ϵ}_{k,s}}^{k,s}X$ (the $k$-th symmetric tensor product of $X$ endowed with the injective symmetric tensor norm) is \emph{never} an $M$-ideal in ${\widehat{⨂}}_{{ϵ}_{k,s}}^{k,s}Y$. This result marks up a difference with the behavior of non-symmetric tensors since, when $X$ is an $M$-ideal in $Y$, it is known that ${\widehat{⨂}}_{{ϵ}_k}^{k}X$ (the $k$-th tensor product of $X$ endowed with the injective tensor norm) is an $M$-ideal in ${\widehat{⨂}}_{{ϵ}_k}^{k}Y$. Nevertheless, if $X$ is Asplund, we prove that every integral $k$-homogeneous polynomial in $X$ has a unique extension to $Y$ that preserves the integral norm. We explicitly describe this extension. We also give necessary and sufficient conditions (related with the continuity of the Aron-Berner extension morphism) for a fixed $k$-homogeneous polynomial $P$ belonging to a maximal polynomial ideal $\text{Q}{(}^{k}X)$ to have a unique norm preserving extension to $\text{Q}{(}^k{X}^{\ast \ast })$. To this end, we study the relationship between the bidual of the symmetric tensor product of a Banach space and the symmetric tensor product of its bidual and show (in the presence of the BAP) that both spaces have `the same local structure’. Other applications to the metric and isomorphic theory of symmetric tensor products and polynomial ideals are also given.