THE NORMING SET of A SYMMETRIC 3-LINEAR FORM on the PLANE with the l₁-NORM

An element (x1, . . ., xn) ∈ Eⁿ is called a norming point of T ∈ L(ⁿE) if kx1k = ··· = kxnk = 1 and |T(x1, . . ., xn)| = kTk, where L(ⁿE) denotes the space of all continuous n-linear forms on E. For T ∈ L(ⁿE), we define Norm(T) = n (x1, . . ., xn) ∈ Eⁿ : (x1, . . ., xn) is a norming point of T ^o . Norm(T) is called the norming set of T. We classify Norm(T) for every T ∈ Ls(³l₁²).