Let $\{\theta_n\}_{n=1}^\infty$ be a sequence of words. If there exists a positive integer $n$ such that $\theta_m(G)=1$ for every $m\geq n$, then we say that $G$ satisfies (*) and denote the class of all groups satisfying (*) by $\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$. If for every proper subgroup $K$ of $G$, $K\in \mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$ but $G\notin\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$, then we call $G$ a minimal non-$\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$-group. Assume that $G$ is an infinite locally finite group with trivial center and $\theta_i(G)=G$ for all $i\geq 1$. In this case we mainly prove that there exists a positive integer $t$ such that for every proper normal subgroup $N$ of $G$, either $\theta_t(N)=1$ or $\theta_t(C_G(N))=1$. We also give certain useful applications of the main result.

Let $\{\theta_n\}_{n=1}^\infty$ be a sequence of words. If there exists a positive integer $n$ such that $\theta_m(G)=1$ for every $m\geq n$, then we say that $G$ satisfies (*) and denote the class of all groups satisfying (*) by $\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$. If for every proper subgroup $K$ of $G$, $K\in \mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$ but $G\notin\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$, then we call $G$ a minimal non-$\mathfrak{X}_{\{\theta_n\}_{n=1}^\infty}$-group. Assume that $G$ is an infinite locally finite group with trivial center and $\theta_i(G)=G$ for all $i\geq 1$. In this case we mainly prove that there exists a positive integer $t$ such that for every proper normal subgroup $N$ of $G$, either $\theta_t(N)=1$ or $\theta_t(C_G(N))=1$. We also give certain useful applications of the main result.