A treatise on the analytical geometry by Casey J. PDF

By Casey J.

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6. Arguments of the same kind prove that the assignment just defined is independent of the several choices involved (α, ω and the extension of F), and that it defines an action. 2] 31 or, equivalently we could have chosen the one described as follows o ω o ω F ω G ω G cc ω G  cc    ω ω ω cccω  cc  cc  ω     1 o ω G2  1 ✷ In a similar way, we may define an action of π1 (X, x) on πn (X, x). In our case, this gives an action of π1 (A, x) on both πn (A, x) and πn (X, x). Moreover, all maps in the homotopy exact sequence are maps of π1 (A, x)-groups.

The main examples of groupoids in his book are equivalence relations and holonomy groupoids of foliations. On the other hand, in books on category theory the role of groupoids is often fundamental (see for example Mac Lane and Moerdijk [MLM96]). In foliation theory, which is a part of differential topology and geometry, the notion of holonomy groupoid is widely used. For surveys of the use of groupoids, see [Bro87, Hig71, Wei01, Mac05, Bro07]. Groupoids have been used extensively by Ehresmann, [Ehr80].

Crossed modules had occurred earlier in other places. In the mid 1960s the great school of Grothendieck in Paris had considered sets with two structures, that of group and of groupoid, and had proved these were equivalent to crossed modules. However this result was not published, and so was known only to a restricted group of people. It is now clear that once one moves to higher version of groupoids, the presence of crossed modules is inevitable, and is an important part of the theory and applications.

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A treatise on the analytical geometry by Casey J.


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