Helmholtz’s equations provide the motion of a system of N vortices which describes a planar incompressible fluid. A relative equilibrium is a particular solution of these equations for which the distances between the particles are invariant during the motion. In this article, we are interested in relative equilibria formed of concentric regular polygons of vortices. We show that in the case of ...

Suppose n is a fixed positive integer. We introduce the relative n-th non-commuting graph ΓH,G, associated to the nonabelian subgroup H of group G. The vertex set is G \ C H,G in which C H,G = {x ∈ G : [x, y] = 1 and [x, y] = 1 for all y ∈ H}. Moreover, {x, y} is an edge if x or y belong to H and xy 6= yx or xy 6= yx. In fact, the relative n-th commutativity degree, Pn(H,G) the probability that...