We give a rigorous geometric proof of the Murakami-Yano formula for the volume of a hyperbolic tetrahedron. In doing so, we are led to consider generalized hyperbolic tetrahedra, which are allowed to be non-convex, and have vertices ‘beyond infinity’; and we uncover a group, which we call 22.5K, of 23040 = 30 · 12 · 2 scissors-class-preserving symmetries of the space of (suitably decorated) gen...

Let G be a finite group given in one of the forms listed in the title with period 2d and X(n) an n-dimensional CW -complex with the homotopy type of an n-sphere. We study the automorphism group Aut (G) to compute the number of distinct homotopy types of orbit spaces X(2dn − 1)/μ with respect to free and cellular G-actions μ on all CW complexes X(2dn−1). At the end, the groups E(X(2dn−1)/μ) of s...

We construct a nonuniform lattice and an infinite family of uniform lattices in the automorphism group of a hyperbolic building with all links a fixed finite building of rank 2 associated to a Chevalley group. We use complexes of groups and basic facts about spherical buildings.

There are only finitely many locally projective regular polytopes of type {5, 3, 5}. They are covered by a locally spherical polytope whose automorphism group is J1 × J1 × L2(19), where J1 is the first Janko group, of order 175560, and L2(19) is the projective special linear group of order 3420. This polytope is minimal, in the sense that any other polytope that covers all locally projective po...

We prove that the automorphism group of a putative binary self-dual doublyeven [72,36,16] code is solvable. Moreover, its order is 5, 7, 10, 14, 56, or a divisor of 72.

Suppose that a group G acts transitively on the points of a nonDesarguesian plane, P. We prove first that the Sylow 2-subgroups of G are cyclic or generalized quaternion. We also prove that P must admit an odd order automorphism group which acts transitively on the set of points of P. 1 MSC(2000): 20B25, 51A35.

Based on Donaldson’s method, we prove that, for an integral Kähler class, when there is a Kähler metric of constant scalar curvature, then it minimizes the K-energy. We do not assume that the automorphism group is discrete.

If (W, S) is a right-angled Coxeter group, then Aut(W ) is a semidirect product of the group Aut◦(W ) of symmetric automorphisms by the automorphism group of a certain groupoid. We show that, under mild conditions, Aut◦(W ) is a semidirect product of Inn(W ) by the quotient Out◦(W ) = Aut◦(W )/Inn(W ). We also give sufficient conditions for the compatibility of the two semidirect products. When...

We present a class of transitive BLT-sets that contains several known examples and at least one new example over the field with 41 elements with automorphism group Z21 : (Z2 × Z2). This example is sporadic in the sense that it does not fall into any of the known infinite families of BLT-sets.

It is proved that the automorphism group of a semigroup being an inflation of its proper subsemigroup decomposes into a semidirect product of two groups one of which is a direct sum of full symmetric groups.