| Poincaré-Birkhoff fixed-point theorem [´pwän·kǝ¦rā `bǝrk´hȯf ¦fikst ¦pȯint `thir·ǝm] MATHEMATICS The theorem that a bijective, continuous, area-preserving mapping of the ring between two concentric circles onto itself that moves one circle in the positive sense and the other in the negative sense has at least two fixed points. |