Symplectic rational blowdown of a minus-four sphere
= Symplectic rational blowdown of a minus-four sphere
A symplectic sphere of self-intersection $-4$ can be rationally blown down by taking the <symplectic sum> with $\mathbb{CP}^2$ along the sphere and a smooth conic of self-intersection $+4$. Equivalently, its disk-bundle neighborhood is replaced by the rational ball $\mathbb{CP}^2\setminus\nu(Q)$ whose boundary is the lens space $L(4,1)$.