Skew-Hermitian operators in real Banach spaces of self-adjoint compact operators

Abstract: Let $\mathcal H$ be a complex infinite-dimensional separable Hilbert space, and let $\mathcal K(\mathcal H)$ be the $C^*$-algebra of compact linear operators in $\mathcal H$. Let $(E,|\cdot|E)$ be a symmetric sequence space. If ${\mu(n,x)}$ are the singular values of $x\in\mathcal K(\mathcal H)$, let $\mathcal C_E={x\in\mathcal K(\mathcal H): {\mu(n,x)}\in E}$ with $|x|{\mathcal C_E}=|{\mu(n,x)}|E$, $x\in\mathcal C_E$, be the Banach ideal of compact operators generated by $E$. Let $\mathcal C_E^h={x\in\mathcal C_E : x=x^*}$ be the real Banach subspace of self-adjoint operators in $(\mathcal C_E, |\cdot|{\mathcal C_E})$. We show that in the case when $\mathcal C_E$ is a separable or perfect Banach symmetric ideal, $\mathcal C_E \neq \mathcal C_{l_2}$, for any skew-Hermitian operator $H\colon\mathcal C_E^h \to \mathcal C_E^h$ there exists self-adjoint bounded linear operator $a$ in $\mathcal H$ such that $H(x)=i(xa - ax)$ for all $x\in\mathcal C_E^h$.