Parametric connected sum
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 1 Introduction
Parametric connected sum is an operation on compact connected n-manifolds and equipped with codimension 0-embeddings and of a compact connected manifold . It generalises the usual connected sum operation which is the special case when is the -disc. The parametric connected sum operation is more complicated than the usual connected sum operation since the isotopy classes of the embeddings of into may be significantly more complicated than the isotopy classes of embeddings of n-discs need for connected sum: these last are determined by (local) orientations.
 2 Connected sum along k-spheres
We say above that to define connected sum for connected k-manifolds and it is sufficient to equip them with an isotopy class of embeddings of the k-disc. Moreover, the disjoint union is the unique thickening of . This motivates the following
Defintion 2.1. A manifold with an -thickening, an -thickened manifold for short, is a pair where is a compact connected manifold and is an embedding.
Defintion 2.2. Let and by -thickened manifolds. Define
where is defined via the embeddings and .
It is clear that we have the following
Observation 2.3. The diffeomorphism type of depends only upon the the isotopy classes of the embeddings and (which of course includes the diffeomorphism types of and ).
 2.1 Applications
The analogue of such a construction for embeddings, the -parametric connected sum of embeddings, is used
- to define, for , a group stucture on the set of (smooth or PL) isotopy classes of embeddings [Skopenkov2006], \S3.4, [Skopenkov2006a], \S3, [Skopenkov2015a].
- to construct an action of this group on the set of isotopy classes of embeddings of certain -manifolds into [Skopenkov2014], 1.2.
- to estimate the set of isotopy classes of embeddings [Cencelj&Repovš&Skopenkov2007], [Cencelj&Repovš&Skopenkov2008], [Skopenkov2007], [Skopenkov2010], [Skopenkov2015], [Skopenkov2015a], [Crowley&Skopenkov2016] and unpublished paper [Crowley&Skopenkov2016a].
 3 Parametric connected sum along thickenings
Let be a stable fibred vector bundle. A foundational theorem of modified surgery is
In particular, has the structure of an abelian group. The question of whether there is a geometric definition of this group structure is taken up in [Kreck1985, Chapter 2, pp 25-6] where it is shown how to use parametric connected sum along thickenings to define an addition of stable diffeomorphism classes of closed --manifolds. This is described in more detail (for ) in [Kreck2016, Section 6] and uses Wall's theory of thickenings, developed in [Wall1966a]. More precisely, it depends on Wall's embedding theorem [Wall1966a, p 76] for the existence of (unique up to concordance) embedded thickenings of the -skeleton of , and Wall's classification of thickenings in the stable range [Wall1966a, Proposition 5.1] to ensure that two such embedded thickenings are diffeomorphic as -manifolds, so that one may cut out their interiors and glue the resulting -manifolds along the boundaries of the embedded thickenings. The special case of is discussed separately in [Kreck2016, Section 5] under the name "connected sum along the -skeleton".
 4 References
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