对于任意一个有限群G,令π(G)表示由它的阶的所有素因子所构成的集合.该文构建一种与之相关的简单图,称之为素图,记作Г(G).该图的顶点集合是π(G),图中两顶点p,q相连(记作p~q)的充要条件是群G恰有Pq阶元[7 5].令π(G)={p1,P2,…,ps}.对于任意p∈π(G),令deg(p)∶=|{q∈π(G)|在素图Г(G)中,p~q}|,并称之为顶点p的度数.同时,我们定义D(G)∶=(deg(p1),deg(p2),…,deg(ps)),其中p1<P2<…<ps,并称之为群G的素图度数序列.若存在k个互不同构的群与群G具有相同的群阶和素图度数序列,则称群G是可k-重OD-刻画的.特别地,可1-重OD-刻画的群也称为可OD-刻画的群[11].在该文中,引入一个新的引理并证明了特殊射影线性群L15(2)是可OD-刻画的.作为一个推论,得到L15(2)是可OG-刻画的.该方法也可适用于其它一些具体的有限单群.
If G is a finite group, we define its prime graph F(G) as follows. The vertices of F(G) are the primes dividing the order of G and two distinct vertices p, q are joined by an edge, denoted by p N q, if and only if there is an element in G of order pq (see [7, 15]). Ozk Assume IGI = plp2...pk with primes pl 〈 P2 〈 〈 Pk and natural numbers ai. For p {Pl,Pa, ,Pk}, define deg(p) := [{q e 7r(G)[q p}[, which is called the degree of p. We also define D(G) := (deg(pl), deg(p2),', deg(pk)), which is called the degree pattern of the group G. We say a group G is t-fold OD-characterizable if there exist exactly t non- isomorphic finite groups M such that IMI = IGI and D(M) D(G) (see [11]). In particular, a 1-fold OD-characterizable group is simply called an OD-characterizable group. In the present paper, we prove that the projective special linear group L15 (2) is OD-eharacterizable by a newly introduced lemma to deal with its connected prime graph. As a consequence of this result, we obtain that L15(2) is OG-eharaeterizable.