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A based self-map of the punctured disk inducing the identity on the fundamental group is based-homotopic to the identity
Statement
Let be continuous with and on . Then is homotopic to the identity relative to . No choice principle is used.
Facts & Assumptions
Given: the based map and .
The truncated flower is a based deformation retract of ; collapsing its tether tree to is a based homotopy equivalence , where is a wedge of circles (The standard flower is a deformation retract with free meridian basis, CW quotients and collapse of a contractible subcomplex, The wedge of a family of pointed spaces).
Based maps induce homomorphisms, composition is functorial, and a based homotopy induces equal maps of fundamental groups (Induced fundamental-group maps are well defined, functorial and invariant under based homotopy, A retract induces an injection on fundamental groups, and a deformation retract induces an isomorphism).
Equal based-loop classes admit homotopies fixing the basepoint (Based loops and the fundamental group, Homotopies of continuous maps, homotopies relative to a subspace, and path homotopies relative to the endpoints).
Proof
Passing to an actual wedge. Combine the deformation retraction with the collapse equivalence of [F1]. They give based maps , with and relative to their basepoints. For , [F2] and imply .
Homotoping the wedge map. Restrict to each actual circle summand of . Its based-loop class equals that summand's standard generator because . By [F3], it has a based homotopy to that summand's inclusion. The finitely many homotopies agree at the wedge vertex at every time and hence glue continuously on the finite quotient . They give relative to the vertex.
Returning to the punctured disk. Compose the based homotopies to obtain , all relative to . For the wedge is a point and the same argument is the based contraction of the disk. This is a homotopy of maps on ; it claims no extension to any puncture. Only finitely many based-loop homotopies and the specified finite graph equivalences occur, so no choice principle is used.
Depends on
- The standard flower is a deformation retract with free meridian basis
- Homotopies of continuous maps, homotopies relative to a subspace, and path homotopies relative to the endpoints
- Induced fundamental-group maps are well defined, functorial and invariant under based homotopy
- The wedge of a family of pointed spaces
- Based loops and the fundamental group
- Free group on a set of generators
- A retract induces an injection on fundamental groups, and a deformation retract induces an isomorphism
- CW quotients and collapse of a contractible subcomplex
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- Allen Hatcher, Algebraic Topology, Proposition 1B.9 and section 1.A (maps from wedges of spheres, aspherical graphs) (standard reference, not scraped)
- Juan Gonzalez-Meneses, Basic results on braid groups, section 1.6, printed pp. 8-10 (standard reference, not scraped)