Circle actions and Suspension operations on Smooth manifolds

Let $M$ be a smooth manifold with $\dim M\geq 3$ and a base point $x_{0}$. Surgeries along the oriented circle $S^{1}\times \{x_{0}\}$ on the product $S^{1}\times M$ yields two manifolds $\Sigma _{0}M$ and $\Sigma _{1}M$, called the suspensions of $M$. The suspension operations $\Sigma _{i}$ play a basic role in the construction and classification of the smooth manifolds which admit free $S^{1}$-actions. We illustrate this by a number of applications.