A block-theory-free characterization of
Let be a finite group and the set of numbers of elements with the same order in . In this paper, we prove that a finite group is isomorphic to , where is one of the Mathieu groups, if and only if the following hold: (1) , (2) .
Let be a finite group and let denote the set of conjugacy class sizes of . Thompson’s conjecture states that if is a centerless group and is a non-abelian simple group satisfying , then . In this paper, we investigate a variation of this conjecture for some symmetric groups under a weaker assumption. In particular, it is shown that if and only if and has a special conjugacy class of size , where is a prime number. Consequently, if is a centerless group with , then .