Isotopy
1 Definition
Two embeddings are said to be isotopic (see [Skopenkov2006], Figure 1.1), if there exists a homeomorphism onto (an ambient isotopy) such that
- for each
- for each and
- for each
An ambient isotopy is also a homotopy or a family of homeomorphisms generated by the map in the obvious manner.
Evidently, isotopy is an equivalence relation on the set of embeddings of into .
2 Other equivalence relations
Ambient isotopy is a stronger equivalence relation than non-ambient isotopy, isoposition, concordance, bordism, etc.
Two embeddings are called isotopic, if there exists an embedding such that
- ,
- for each and
- for each .
In the DIFF category or for in the PL or TOP category isotopy implies ambient isotopy [Hudson&Zeeman1964], [Hudson1966], [Akin1969], [Edwards1975], \S7. For this is not so: e.g., any knot is non-ambiently PL isotopic to the trivial one, but not necessarily ambiently PL isotopic to it.
Two embeddings are said to be (orientation preserving) , if there is an (orientation preserving) homeomorphism such that .
For embeddings into PL orientation preserving isoposition is equivalent to PL isotopy (the Alexander-Guggenheim Theorem) [Rourke&Sanderson1972], 3.22.
Two embeddings are said to be concordant if there is a homeomorphism (onto) (which is called a ) such that
- for each and
- for each .
The definition of - is analogously obtained from that of non-ambient isotopy by dropping the last condition of level-preservation.
In the DIFF category or for in the PL or TOP category concordance implies ambient concordance and isotopy [Lickorish1965], [Hudson1970], [Hudson&Lickorish1971]. (This is not so in the PL or TOP category for codimension 2.) This result allows a reduction of the problem of isotopy to the relativized problem of embeddability.