Para-linearity as the nonassociative counterpart of linearity

Abstract

In an octonionic Hilbert space $H$, the octonionic linearity is taken to fail for the maps induced by the octonionic inner products, and it should be replaced with the octonionic para-linearity. However, to introduce the notion of the octonionic para-linearity we encounter an insurmountable obstacle. That is, the axiom $$\left\langle pu ,u\right\rangle=p\left\langle u ,u\right\rangle$$ for any octonion $p$ and element $u\in H$ introduced by Goldstine and Horwitz in 1964 can not be interpreted as a property to be obeyed by the octonionic para-linear maps. In this article, we solve this critical problem by showing that this axiom is in fact non-independent from others. This enables us to initiate the study of octonionic para-linear maps. We can thus establish the octonionic Riesz representation theorem which, up to isomorphism, identifies two octonionic Hilbert spaces with one being the dual of the other. The dual space consists of continuous left \almost linear functionals and it becomes a right $Ø$-module under the multiplication defined in terms of the second associators which measures the failure of $Ø$-linearity. This right multiplication has an alternative expression $${(f\odot p)(x)}=pf(p^{-1}x)p,$$ which is a generalized Moufang identity. Remarkably, the multiplication is compatible with the canonical norm, i.e., $$\fsh{f\odot p}=\fsh{f}\abs{p}.$$ Our final conclusion is that para-linearity is the nonassociative counterpart of linearity.