On Schatten-von Neumann class properties of pseudo-differential   operators. Cordes' lemma
  We prove an extended version of Cordes' lemma concerning trace-class properties of some special pseudo-differential operators. This version of Cordes' lemma is used to improve the results in \cite{Arsu} concerning the Schatten-class properties of pseudo-differential operators in the $(X,\tau) $-quantization. Here $X$ is an $n$ dimensional vector space and $\tau $ is an endomorphism of $X$. 
Gruia Arsu