Several Quantitative Characterizations of Some Specific Groups

Let $G$ be a finite group and let $pi(G)={p_1, p_2, ldots, p_k}$ be the set of prime divisors of $|G|$ for which $p_1<p_2<cdots<p_k$. The Gruenberg-Kegel graph of $G$, denoted ${rm GK}(G)$, is defined as follows: its vertex set is $pi(G)$ and two different vertices $p_i$ and $p_j$ are adjacent by an edge if and only if $G$ contains an element of order $p_ip_j$. The degree of a vertex $p_i$ in ${rm GK}(G)$ is denoted by $d_G(p_i)$ and the $k$-tuple $D(G)=left(d_G(p_1), d_G(p_2), ldots, d_G(p_k)right)$ is said to be the degree pattern of $G$. Moreover, if $omega subseteq pi(G)$ is the vertex set of a connected component of ${rm GK}(G)$, then the largest $omega$-number which divides $|G|$, is said to be an order component of ${rm GK}(G)$. We will say that the problem of OD-characterization is solved for a finite group if we find the number of pairwise non-isomorphic finite groups with the same order and degree pattern as the group under study. The purpose of this article is twofold. First, we completely solve the problem of OD-characterization for every finite non-abelian simple group with orders having prime divisors at most 29. In particular, we show that there are exactly two non-isomorphic finite groups with the same order and degree pattern as $U_4(2)$. Second, we prove that there are exactly two non-isomorphic finite groups with the same order components as $U_5(2)$.