On $\BCI$-groups and $\CI$-groups

Let $G$ be a finite group and $S$ be a subset of $G.$ A bi-Cayley graph $\BCay(G,S)$ is a simple and an undirected graph with vertex-set $G\times\{1,2\}$ and edge-set $\{\{(g,1),(sg,2)\}\mid g\in G, s\in S\}$. A bi-Cayley graph $\BCay(G,S)$ is called a $\BCI$-graph if for any bi-Cayley graph $\BCay(G,T)$, whenever $\BCay(G,S)\cong\BCay(G,T)$ we have $T=gS^σ$ for some $g\in G$ and $σ\in\Aut(G).$ A group $G$ is called a $\BCI$-group if every bi-Cayley graph of $G$ is a $\BCI$-graph. In this paper, we showed that every $\BCI$-group is a $\CI$-group, which gives a positive answer to a conjecture proposed by Arezoomand and Taeri in \cite{arezoomand1}. Also we proved that there is no any non-Abelian $4$-$\BCI$-simple group. In addition all $\BCI$-groups of order $2p$, $p$ a prime, are characterized.

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