| Title | The necessity of the star-shaped condition on Ding's version of the Poincaré-Birkhoff theorem |
| Publication Type | Unpublished |
| Year of Publication | 2006 |
| Authors | Martins R, Ureña AJ |
| Series Title | Preprint |
| Abstract | W-Y. Ding proved a generalization of the Poincaré-Birkhoff fixed point theorem not requiring that the boundary curves of the anular region be circles and invariant. However the inner boundary is required to be star-shaped with respect to the origin. We construct a counterexample that shows that the star-shaped condition is necessary. |
| URL | http://www.dm.fct.unl.pt/sites/www.dm.fct.unl.pt/files/preprints/2006/16_06.pdf |