IMMERSED SPHERES IN INDEFINITE 4-MANIFOLDS
非正(负)定四维流形中的浸入球

The minimal number of double points of immersed 2-spheres in indefinite 4-manifoldsCP~2 #m \bar{CP^2} is studied and a lower bound is given.Combining with the action of orthogonalgroups,a complete answer is given to whether a 2-dimensional homology class with positivesquare can be represented by an embedded sphere for m=2 or 3.The representability of someinteresting cases is also proved for general m.