Desmond MacHale's intriguing article [2] neatly illustrates one mode of interplay between proof and example in group theory by describing the commonly used inductive technique of minimal counterexamples. In this Note, we use some ideas from group theory and linear algebra to lead the reader through a geometric and algebraic construction of one of the minimal counterexamples sought by MacHale. Recall that a group is called nilpotent if each of its Sylow subgroups is normal. One of MacHale's list of known-to-be-false conjectures is this: if G is a finite group having a fixed-pointfree automorphism 4, then G is nilpotent. The true theorem on which the conjecture is based assumes that 4 is of prime order; it was established by Thompson in his dissertation in 1959 [4]. His proof so excited the mathematical community that even the New York Times reported his result and the reaction to it [3]. MacHale indicates that [1, p. 336] contains details of a counterexample to the conjecture with IGI = 147 = 72 3 and 1j1 = 4. In our minimal counterexample to be constructed, I4 = 6 and IGI = 48 = 24 43. In what follows, we use some standard conventions of notation in group theory. In particular, we write operators on the right and denote both the action of a group homomorphism and conjugation using superscripts. Recall that if H and K are subgroups of a group G such that G = HK, H n K = 1, and H is normal in G, then G is called the semidirect product of H by K. In this situation, since H is normal in G, each element of K acts via conjugation as an automorphism of H. Of particular interest here is the situation in which each nontrivial element of K acts as a nontrivial (i.e., nonidentity) automorphism on H. In this case K is isomorphic to a subgroup of Aut H, the group of all automorphisms of H. A well-known geometric example leads directly to an important link between group theory and geometry. If A is the group of rotations of a regular tetrahedron, then A contains eight elements of order 3, three of order 2, and an identity element. Each element of order 3 is a 120?rotation of the tetrahedron about an axis passing through one of the 4 vertices and perpendicular to the opposite face. Each element of order 2 is a rotation of 1800 about one of three axes joining the midpoints of two nonadjacent edges of the tetrahedron. (See FIGURE 1.) If the vertices are labeled, and each rotation is identified with the permutation of the labels it produces, this identification provides a natural isomorphism between A and A4, the group of even permutations on 4 letters. Thus it is possible to use the algebra of permutations or geometry to analyze A.
Read full abstract