A useful method of
constructing-3-dimensional manifolds is to remove the interior of a tubular
neighbourhood V ⊂ S3 of a knot K in the 3-sphere and sew it back differently, via a
homeomorphism h : ∂V → ∂V . This surgery construction, due to M. Dehn, yields the
manifold
where x ∈ ∂V ⊂ V is identified with h(x) ∈ ∂V ⊂ S3 − int V . For example
surgery along a trivial knot yields, for various choices of h, exactly the
class of lens spaces L(p,q), including the extreme cases L(1,0)≅S3 and
L(0,1)≅S2 × S1.
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