The norming set of a symmetric 3-linear form on the plane with the $l_1$-norm

Abstract

An element $(x_1, \ldots, x_n)\in E^n$ is called a {\em norming point} of $T\in {\mathcal L}(^n E)$ if $\|x_1\|=\cdots=\|x_n\|=1$ and
$|T(x_1, \ldots, x_n)|=\|T\|$, where ${\mathcal L}(^n E)$ denotes the space of all continuous $n$-linear forms on $E$.
For $T\in {\mathcal L}(^n E)$, we define $${Norm}(T)=\Big\{(x_1, \ldots, x_n)\in E^n: (x_1, \ldots, x_n)~\mbox{is a norming point of}~T\Big\}.$$
${Norm}(T)$ is called the {\em norming set} of $T$. We classify ${Norm}(T)$ for every $T\in {\mathcal L}_s(^3 l_{1}^2)$.