# Loop theorem

Loop theorem THE LOOP THEOREM

MSC 57M35

In the topology^{} of 3-manifolds, the loop theorem is generalization^{} of an ansatz discovered by Max Dehn (The Dehn’s Lemma),
who saw that if a continuous map from a 2-disk to a 3-manifold whose restriction^{} to the boundary’s disk has no singularities,
then it exists another embedding^{} whose restriction to the boundary’s disk is equal to the boundary’s restriction original map.

The following statement called the Loop Theorem is a version from J. Stalling, but written in W. Jaco’s book.

Let $M$ be a three-manifold and let $S$
be a connected surface in $\mathrm{\partial}\mathit{}M$. Let $N\mathrm{\subset}{\pi}_{\mathrm{(}}M\mathrm{)}$ be a normal subgroup^{}.
Let $f\mathrm{:}{D}^{\mathrm{2}}\mathrm{\to}M$
be a continuous map such that $f\mathit{}\mathrm{(}\mathrm{\partial}\mathit{}{D}^{\mathrm{2}}\mathrm{)}\mathrm{\subset}S$
and $\mathrm{[}f\mathrm{|}\mathrm{\partial}{D}^{\mathrm{2}}\mathrm{]}\mathrm{\notin}N$.

Then there exists an embedding
$g\mathrm{:}{D}^{\mathrm{2}}\mathrm{\to}M$ such that
$g\mathit{}\mathrm{(}\mathrm{\partial}\mathit{}{D}^{\mathrm{2}}\mathrm{)}\mathrm{\subset}S$
and
$\mathrm{[}g\mathrm{|}\mathrm{\partial}{D}^{\mathrm{2}}\mathrm{]}\mathrm{\notin}N$,

The proof is a clever construction due to C. Papakyriakopoulos about a sequence (a tower) of covering spaces. Maybe the best detailed redaction is due to A. Hatcher. But in general, accordingly to Jaco’s opinion, ”… for anyone unfamiliar with the techniques of 3-manifold-topology and are here to gain a working knowledge for the study of problems in this area…, there is no better place to start.”

==References==

W. Jaco, Lectures on 3-manifolds topology, A.M.S. regional conference series in Math 43.

J. Hempel, 3-manifolds, Princeton University Press 1976.

A. Hatcher, Notes on 3-manifolds,

Title | Loop theorem |
---|---|

Canonical name | LoopTheorem12 |

Date of creation | 2013-03-11 19:24:08 |

Last modified on | 2013-03-11 19:24:08 |

Owner | juanman (12619) |

Last modified by | (0) |

Numerical id | 1 |

Author | juanman (0) |

Entry type | Definition |