A Van Kampen type theorem for coincidences

AbstractThe Nielsen coincidence theory is well understood for a pair of maps (f,g):Mn→Nn where M and N are compact manifolds of the same dimension greater than two. We consider coincidence theory of a pair (f,g):K→Nn, where the complex K is the union of two compact manifolds of the same dimension as Nn. We define a number N(f,g:K1,K2) which is a homotopy invariant with respect to the maps. This number is certainly a lower bound for the number of coincidence points, and we prove a minimizing theorem with respect to this number. Finally, we consider the case where the target is a Jiang space and we obtain a nicer description of N(f,g:K1,K2) in terms of the Nielsen coincidence numbers of the maps restricted to the subspaces K1, K2