Fundamental Group

In mathematics, more specifically algebraic topology, the fundamental group (defined by Henri Poincaré in his article Analysis Situs, published in 1895) is a group associated to any given pointed topological space that provides a way of determining when two paths, starting and ending at a fixed base point, can be continuously deformed into each other. Intuitively, it records information about the basic shape, or holes, of the topological space. The fundamental group is the first and simplest of the homotopy groups. It is a topological invariant: homeomorphic topological spaces have the same fundamental group.

Fundamental groups can be studied using the theory of covering spaces, since a fundamental group coincides with the group of deck transformations of the associated universal covering space. Its abelianisation can be identified with the first homology group of the space. When the topological space is homeomorphic to a simplicial complex, its fundamental group can be described explicitly in terms of generators and relations.

Historically, the concept of fundamental group first emerged in the theory of Riemann surfaces, in the work of Bernhard Riemann, Henri Poincaré and Felix Klein, where it describes the monodromy properties of complex functions, as well as providing a complete topological classification of closed surfaces.

Read more about Fundamental GroupIntuition, Definition, Examples, Functoriality, Fibrations, Relationship To First Homology Group, Universal Covering Space, Edge-path Group of A Simplicial Complex, Realizability, Related Concepts

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... is a sketch of the proof of Grushko's theorem based on the use of foldings techniques for groups acting on trees (see for complete proofs using this argument) ... Realize G as the fundamental group of a graph of groups Y which is a single non-loop edge with vertex groups A and B and with the trivial edge group ... Let F=F(x1...xn) be the free group with free basis x1...xn and let φ0F → G be the homomorphism such that φ0(xi)=gi for i=1...n ...
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... For example, the fundamental group of the affine line is not topologically finitely generated ... The tame fundamental group of some scheme U is a quotient of the usual fundamental group of U which takes into account only covers that are tamely ramified along D, where X is some compactification and D ... For example, the tame fundamental group of the affine line is zero ...

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