Endomorphisms of the 2-adic ring $C^*$-algebra and its Weyl group
In this paper, we establish a one-to-one correspondence between a natural monoid and the endomorphisms of the $2$-adic ring $C^*$-algebra $\mathcal{Q}_2$. As a consequence, we classify endomorphisms of $\mathcal{Q}_2$ with prescribed images and derive several criteria for the uniqueness of extensions of endomorphisms of a canonical copy of the Cuntz algebra $\mathcal{O}_2$ inside $\mathcal{Q}_2$ to endomorphisms of $\mathcal{Q}_2$. Moreover, we construct an example of an extendable automorphism of $\mathcal{O}_2$ that is not a composition of the flip-flop automorphism, the gauge automorphisms, and inner automorphisms, thereby providing negative answers to certain open questions. Using this explicit construction, we also show that the canonical image of $\operatorname{Aut}(\mathcal{Q}_2,\mathcal{O}_2)$ in the outer automorphism group $\operatorname{Out}(\mathcal{Q}_2)$ is non-abelian. Finally, we characterize the automorphisms of $\mathcal{Q}_2$ that globally preserve $C^*(u)$, and completely describe the Weyl group $\mathcal{W}(\mathcal{Q}_2, C^*(u))$. Consequently, several related open questions are answered affirmatively.