Sphere bundles and spin (Ex)

For $k\geq2$$<wikitex>; For $k\geq2$, determine the number of distinct $S^k$-bundles over $S^2$. For $m\geq 4$, let $S^1\hookrightarrow M^m$ be a nullhomotopic embedding. For $M$ spin, show that there is more than one possible effect for a surgery on this embedding. Now suppose $M$ is simply connected. For $M$ not spin, show that the effect of a surgery on this embedding ''is'' uniquely determined (difficult!). </wikitex> == References == {{#RefList:}} [[Category:Exercises]]k\geq2$, determine the number of distinct $S^k$$S^k$-bundles over $S^2$$S^2$.

For $m\geq 4$$m\geq 4$, let $S^1\hookrightarrow M^m$$S^1\hookrightarrow M^m$ be a nullhomotopic embedding. For $M$$M$ spin, show that there is more than one possible effect for a surgery on this embedding.

Now suppose $M$$M$ is simply connected. For $M$$M$ not spin, show that the effect of a surgery on this embedding is uniquely determined (difficult!).