A symplectic isotopy of a Dehn twist on ℂℙn × ℂℙn+1

The complex manifold CPn �~ CPn+1 with symplectic form �Ð�E = �ÐCPn + �E�ÐCPn+1, where �ÐCPn and �ÐCPn+1 are normalized Fubini.Study forms, n �¸ N and �E > 1 a real number, contains a natural Lagrangian sphere L�E. We prove that the Dehn twist along L�E is symplectically isotopic to the identity for all �E > 1. This isotopy can be chosen so that it pointwise fixes a complex hypersurface in CPn �~ CPn+1 and lifts to the blow-up of CPn �~ CPn+1 along a complex n-dimensional submanifold.