sigma, delta constant

If $(\sigma ,\delta )$ is a skew derivation on a ring $R$,then a $(\sigma ,\delta )$-constant is an element $q$ of $R$such that $\sigma (q)=q$ and $\delta (q)=0$.

Note: If $q$ is a $(\sigma ,\delta )$-constant,then it follows that $\sigma (q\cdot x)=q\cdot \sigma (x)$and $\delta (q\cdot x)=q\cdot \delta (x)$ for all $x$ in $R$.