
doi: 10.1070/sm8906
For two set-valued mappings \(\Phi,\Psi: X\multimap Y\) the coincidence set is defined as \(\text{Coin}(\Phi,\Psi):=\{\xi\in X \mid \Phi(\xi)\cap\Psi(\xi)\not=\emptyset\}\). Let \(\alpha>0\) and \(\beta\geq0\). A map \(\Psi:X\multimap Y\) between metric spaces is called an \(\alpha\)-covering if \(B_Y(\Psi(x_0);\alpha r)\subset \Psi(B_X(x_0;r))\) for all \(x_0\in X\) and \(r>0\) where of course, \(B_X\) and \(B_Y\) denote the corresponding closed balls. \(\Psi\) is called \(\beta\)-Lipschitz if \(h_Y(\Phi(x),\Phi(u))\leq\beta\rho_X(x,u)\) for all \(x,u\in X\) where \(h_Y\) denotes the Hausdorff-distance in \(Y\) and \(\rho_X\) the metric in \(X\). Let now be \(\Omega\) be an arbitrary set (finite or infinite) and let \(\Phi:X\multimap Y\) be a closed \(\alpha\)-covering and let \(\Psi:X\multimap Y\) be closed and \(\beta\)-Lipschitz. Assume that \(\alpha>\beta\) and that at least one of \(\Phi\) and \(\Psi\) has a graph which is a complete metric space. The authors use the following rather technical assumption: There is an injective mapping \(\Omega\to X\), \(j\mapsto x_j\), and for each \(j\in\Omega\) there is an \(\epsilon_j>0\) such that for \(i\not=j\) we have that \(\rho(x_i,x_j)\geq\frac{\text{dist}_Y(\Psi(x_i),\Phi(x_i))+\text{dist}_Y(\Psi(x_j),\Phi(x_j))}{\alpha-\beta}+\epsilon_i+\epsilon_j\). Let then \((y_j)_{j\in\Omega}\) be a family in \(Y\) such that \(y_j\in\Psi(x_j)\). Then there exist families \((\xi_j)_{j\in\Omega}\) in \(X\) and \((\eta_j)_{j\in\Omega}\) in \(Y\) such that \((\xi_j,\eta_j)\) belongs to the intersection of the graphs of \(\Phi\) and \(\Psi\), \(\xi_i\not=\xi_j\) for \(i\not=j\), and the cardinality of \(\text{Coinc}(\Psi,\Phi)\) is not smaller than the cardinality of \(\Omega\). There are several results in the same spirit where special consideration is given to the case of normed spaces.
coincidence point, covering mapping, Fixed-point and coincidence theorems (topological aspects), Covering mapping, Coincidence point, 510, Set-valued maps in general topology
coincidence point, covering mapping, Fixed-point and coincidence theorems (topological aspects), Covering mapping, Coincidence point, 510, Set-valued maps in general topology
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