Fundamental group

In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the 
loops contained in the space. It records information about the basic shape, or holes, of the topological space. The fundamental group is the first and simplest homotopy group. The fundamental group is a homotopy invariant—topological spaces that are homotopy equivalent (or the stronger case of homeomorphic) have isomorphic fundamental groups.
Intuition.
Start with a space (for example, a surface), and some point in it, and all the loops both starting and ending at this point—paths that start at this point, wander around and eventually return to the starting point. Two loops can be combined together in an obvious way: travel along the first loop, then along the second.
Two loops are considered equivalent if one can be deformed into the other without breaking. The set of all such loops with this method of combining and this equivalence between them is the fundamental group for that particular space.
History.
Henri Poincaré defined the fundamental group in 1895 in his paper "Analysis situs". The concept emerged in the theory of Riemann surfaces, in the work of Bernhard Riemann, Poincaré, and Felix Klein. It describes the monodromy properties of complex-valued functions, as well as providing a complete topological classification of closed surfaces.
Definition.
Throughout this article, "X" is a topological space. A typical example is a surface such as the one depicted at the right. Moreover, formula_1 is a point in "X" called the "base-point". (As is explained below, its role is rather auxiliary.) The idea of the definition of the homotopy group is to measure how many (broadly speaking) curves on "X" can be deformed into each other. The precise definition depends on the notion of the homotopy of loops, which is explained first.
Homotopy of loops.
Given a topological space "X", a "loop based at formula_1" is defined to be a continuous function (also known as a continuous map)
such that the starting point formula_4 and the end point formula_5 are both equal to formula_1.
A "homotopy" is a continuous interpolation between two loops. More precisely, a homotopy between two loops formula_7 (based at the same point formula_1) is a continuous map
such that
If such a homotopy "h" exists, formula_18 and formula_19 are said to be "homotopic". The relation "formula_18 is homotopic to formula_19" is an equivalence relation so that the set of equivalence classes can be considered:
This set (with the group structure described below) is called the "fundamental group" of the topological space "X" at the base point formula_1. The purpose of considering the equivalence classes of loops up to homotopy, as opposed to the set of all loops (the so-called loop space of "X") is that the latter, while being useful for various purposes, is a rather big and unwieldy object. By contrast the above quotient is, in many cases, more manageable and computable.
Group structure.
By the above definition, formula_24 is just a set. It becomes a group (and therefore deserves the name fundamental "group") using the concatenation of loops. More precisely, given two loops formula_25, their product is defined as the loop
Thus the loop formula_27 first follows the loop formula_28 with "twice the speed" and then follows formula_29 with "twice the speed".
The product of two homotopy classes of loops formula_30 and formula_31 is then defined as formula_32. It can be shown that this product does not depend on the choice of representatives and therefore gives a well-defined operation on the set formula_24. This operation turns formula_24 into a group. Its neutral element is the constant loop, which stays at formula_1 for all times "t". The inverse of a (homotopy class of a) loop is the same loop, but traversed in the opposite direction. More formally,
Given three based loops formula_37 the product
is the concatenation of these loops, traversing formula_28 and then formula_29 with quadruple speed, and then formula_41 with double speed. By comparison,
traverses the same paths (in the same order), but formula_28 with double speed, and formula_44 with quadruple speed. Thus, because of the differing speeds, the two paths are not identical. The associativity axiom
therefore crucially depends on the fact that paths are considered up to homotopy. Indeed, both above composites are homotopic, for example, to the loop that traverses all three loops formula_46 with triple speed. The set of based loops up to homotopy, equipped with the above operation therefore does turn formula_24 into a group.
Dependence of the base point.
Although the fundamental group in general depends on the choice of base point, it turns out that, up to isomorphism (actually, even up to "inner" isomorphism), this choice makes no difference as long as the space "X" is path-connected. For path-connected spaces, therefore, many authors write formula_48 instead of formula_24.
Concrete examples.
This section lists some basic examples of fundamental groups. To begin with, in Euclidean space (formula_50) or any convex subset of formula_51 there is only one homotopy class of loops, and the fundamental group is therefore the trivial group with one element. More generally, any star domain and, yet more generally any contractible space has a trivial fundamental group. Thus, the fundamental group does not distinguish between such spaces.
The 2-sphere.
A path-connected space whose fundamental group is trivial is called simply connected. 
For example, the 2-sphere formula_52 depicted on the right, and also all the higher-dimensional spheres are simply-connected. The figure illustrates a homotopy contracting one particular loop to the constant loop. This idea can be adapted to all loops formula_18 such that there is a point formula_54 that is in the image of formula_55 However, since there are loops such that formula_56 (constructed from the Peano curve, for example), a complete proof requires more careful analysis with tools from algebraic topology, such as the Seifert–van Kampen theorem or the cellular approximation theorem.
The circle.
The circle (also known as the 1-sphere)
is not simply connected. Instead, each homotopy class consists of all loops that wind around the circle a given number of times (which can be positive or negative, depending on the direction of winding). The product of a loop that winds around "m" times and another that winds around "n" times is a loop that winds around formula_58 times. Therefore, the fundamental group of the circle is isomorphic to formula_59 the additive group of integers. This fact can be used to give proofs of the Brouwer fixed point theorem and the Borsuk–Ulam theorem in dimension 2.
The figure eight.
The fundamental group of the figure eight is the free group on two letters. The idea to prove this is as follows: choosing the base point to be the point where the two circles meet (dotted in black in the picture at the right), any loop formula_18 can be decomposed as
where "a" and "b" are the two loops winding around each half of the figure as depicted, and the exponents formula_62 are integers. Unlike formula_63 the fundamental group of the figure eight is "not" abelian: the two ways of composing "a" and "b" are not homotopic to each other:
More generally, the fundamental group of a bouquet of "r" circles is the free group on "r" letters.
The fundamental group of a wedge sum of two path connected spaces "X" and "Y" can be computed as the free product of the individual fundamental groups:
This generalizes the above observations since the figure eight is the wedge sum of two circles.
The fundamental group of the plane punctured at "n" points is also the free group with "n" generators. The "i"-th generator is the class of the loop that goes around the "i"-th puncture without going around any other punctures.
Graphs.
The fundamental group can be defined for discrete structures too. In particular, consider a connected graph , with a designated vertex "v"0 in "V". The loops in "G" are the cycles that start and end at "v0". Let "T" be a spanning tree of "G". Every simple loop in "G" contains exactly one edge in "E" \ "T"; every loop in "G" is a concatenation of such simple loops. Therefore, the fundamental group of a graph is a free group, in which the number of generators is exactly the number of edges in "E" \ "T". This number equals .
For example, suppose "G" has 16 vertices arranged in 4 rows of 4 vertices each, with edges connecting vertices that are adjacent horizontally or vertically. Then "G" has 24 edges overall, and the number of edges in each spanning tree is , so the fundamental group of "G" is the free group with 9 generators. Note that "G" has 9 "holes", similarly to a bouquet of 9 circles, which has the same fundamental group.
Knot groups.
"Knot groups" are, by definition the fundamental group of the complement of a knot "K" embedded in formula_66 For example, the knot group of the trefoil knot is known to be the braid group formula_67 which gives another example of a non-abelian fundamental group. The Wirtinger presentation explicitly describes knot groups in terms of generators and relations based on a diagram of the knot. Therefore knot groups have some usage in knot theory to distinguish between knots: if formula_68 is not isomorphic to some other knot group formula_69 of another knot "K"', then "K" can not be transformed into formula_70 Thus the trefoil knot can not be continuously transformed into the circle (also known as the unknot), since the latter has knot group formula_71. There are, however, knots that can not be deformed into each other, but have isomorphic knot groups.
Oriented surfaces.
The fundamental group of a genus "n" orientable surface can be computed in terms of generators and relations as
This includes the torus, being the case of genus 1, whose fundamental group is
Topological groups.
The fundamental group of a topological group "X" (with respect to the base point being the neutral element) is always commutative. In particular, the fundamental group of a Lie group is commutative. In fact, the group structure on "X" endows formula_48 with another group structure: given two loops formula_18 and formula_19 in "X", another loop formula_77 can defined by using the group multiplication in "X": 
This binary operation formula_79 on the set of all loops is "a priori" independent from the one described above. However, the Eckmann–Hilton argument shows that it does in fact agree with the above concatenation of loops, and moreover that the resulting group structure is abelian.
An inspection of the proof shows that, more generally, formula_48 is abelian for any H-space "X", i.e., the multiplication need not have an inverse, nor does it have to be associative. For example, this shows that the fundamental group of a loop space of another topological space "Y", formula_81 is abelian. Related ideas lead to Heinz Hopf's computation of the cohomology of a Lie group.
Functoriality.
If formula_82 is a continuous map, formula_83 and formula_84 with formula_85 then every loop in "X" with base point formula_1 can be composed with "f" to yield a loop in "Y" with base point formula_87 This operation is compatible with the homotopy equivalence relation and with composition of loops. The resulting group homomorphism, called the induced homomorphism, is written as formula_88 or, more commonly,
This mapping from continuous maps to group homomorphisms is compatible with composition of maps and identity morphisms. In the parlance of category theory, the formation of associating to a topological space its fundamental group is therefore a functor
from the category of topological spaces together with a base point to the category of groups. It turns out that this functor does not distinguish maps that are homotopic relative to the base point: if "f", "g" : "X" → "Y" are continuous maps with "f"("x"0) = "g"("x"0) = "y"0, and "f" and "g" are homotopic relative to {"x"0}, then "f"∗ = "g"∗. As a consequence, two homotopy equivalent path-connected spaces have isomorphic fundamental groups:
For example, the inclusion of the circle in the punctured plane
is a homotopy equivalence and therefore yields an isomorphism of their fundamental groups.
The fundamental group functor takes products to products and coproducts to coproducts. That is, if "X" and "Y" are path connected, then
Abstract results.
As was mentioned above, computing the fundamental group of even relatively simple topological spaces tends to be not entirely trivial, but requires some methods of algebraic topology.
Relationship to first homology group.
The abelianization of the fundamental group can be identified with the first homology group of the space.
A special case of the Hurewicz theorem asserts that the first singular homology group formula_94 is, colloquially speaking, the closest approximation to the fundamental group by means of an abelian group. In more detail, mapping the homotopy class of each loop to the homology class of the loop gives a group homomorphism
from the fundamental group of a topological space "X" to its first singular homology group formula_96 This homomorphism is not in general an isomorphism since the fundamental group may be non-abelian, but the homology group is, by definition, always abelian. This difference is, however, the only one: if "X" is path-connected, this homomorphism is surjective and its kernel is the commutator subgroup of the fundamental group, so that formula_94 is isomorphic to the abelianization of the fundamental group.
Glueing topological spaces.
Generalizing the statement above, for a family of path connected spaces formula_98 the fundamental group formula_99 is the free product of the fundamental groups of the formula_100 This fact is a special case of the Seifert–van Kampen theorem, which allows to compute, more generally, fundamental groups of spaces that are glued together from other spaces. For example, the 2-sphere formula_101 can be obtained by glueing two copies of slightly overlapping half-spheres along a neighborhood of the equator. In this case the theorem yields formula_102 is trivial, since the two half-spheres are contractible and therefore have trivial fundamental group. The fundamental groups of surfaces, as mentioned above, can also be computed using this theorem.
In the parlance of category theory, the theorem can be concisely stated by saying that the fundamental group functor takes pushouts (in the category of topological spaces) along inclusions to pushouts (in the category of groups).
Coverings.
Given a topological space "B", a continuous map 
is called a "covering" or "E" is called a "covering space" of "B" if every point "b" in "B" admits an open neighborhood "U" such that there is a homeomorphism between the preimage of "U" and a disjoint union of copies of "U" (indexed by some set "I"),
in such a way that formula_105 is the standard projection map formula_106
Universal covering.
A covering is called a universal covering if "E" is, in addition to the preceding condition, simply connected. It is universal in the sense that all other coverings can be constructed by suitably identifying points in "E". Knowing a universal covering 
of a topological space "X" is helpful in understanding its fundamental group in several ways: first, formula_48 identifies with the group of deck transformations, i.e., the group of homeomorphisms formula_109 that commute with the map to "X", i.e., formula_110
Another relation to the fundamental group is that formula_111 can be identified with the fiber formula_112 For example, the map
(or, equivalently, formula_114) is a universal covering. The deck transformations are the maps formula_115 for formula_116 This is in line with the identification formula_117 in particular this proves the above claim formula_118
Any path connected, locally path connected and locally simply connected topological space "X" admits a universal covering. An abstract construction proceeds analogously to the fundamental group by taking pairs ("x", γ), where "x" is a point in "X" and γ is a homotopy class of paths from "x"0 to "x". The passage from a topological space to its universal covering can be used in understanding the geometry of "X". For example, the uniformization theorem shows that any simply connected Riemann surface is (isomorphic to) either formula_119 formula_120 or the upper half plane. General Riemann surfaces then arise as quotients of group actions on these three surfaces.
The quotient of an action of a (discrete) group "G" on a simply connected space "Y" has fundamental group 
As an example, the real "n"-dimensional real projective space formula_122 is obtained as the quotient of the "n"-dimensional sphere formula_123 by the antipodal action of the group formula_124 sending formula_125 to formula_126 As formula_123 is simply connected for "n" ≥ 2, it is a universal cover of formula_122 in these cases, which implies formula_129 for "n" ≥ 2.
Lie groups.
Let "G" be a connected, simply connected compact Lie group, for example, the special unitary group SU("n"), and let Γ be a finite subgroup of "G". Then the homogeneous space "X" = "G"/Γ has fundamental group Γ, which acts by right multiplication on the universal covering space "G". Among the many variants of this construction, one of the most important is given by locally symmetric spaces "X" = Γ\"G"/"K", where
In this case the fundamental group is Γ and the universal covering space "G"/"K" is actually contractible (by the Cartan decomposition for Lie groups).
As an example take "G" = SL(2, R), "K" = SO(2) and Γ any torsion-free congruence subgroup of the modular group SL(2, Z).
From the explicit realization, it also follows that the universal covering space of a path connected topological group "H" is again a path connected topological group "G". Moreover, the covering map is a continuous open homomorphism of "G" onto "H" with kernel Γ, a closed discrete normal subgroup of "G":
Since "G" is a connected group with a continuous action by conjugation on a discrete group Γ, it must act trivially, so that Γ has to be a subgroup of the center of "G". In particular π1("H") = Γ is an abelian group; this can also easily be seen directly without using covering spaces. The group "G" is called the "universal covering group" of "H".
As the universal covering group suggests, there is an analogy between the fundamental group of a topological group and the center of a group; this is elaborated at Lattice of covering groups.
Fibrations.
"Fibrations" provide a very powerful means to compute homotopy groups. A fibration "f" the so-called "total space", and the base space "B" has, in particular, the property that all its fibers formula_131 are homotopy equivalent and therefore can not be distinguished using fundamental groups (and higher homotopy groups), provided that "B" is path-connected. Therefore, the space "E" can be regarded as a "twisted product" of the base space "B" and the fiber formula_132 The great importance of fibrations to the computation of homotopy groups stems from a long exact sequence
provided that "B" is path-connected. The term formula_134 is the second homotopy group of "B", which is defined to be the set of homotopy classes of maps from formula_101 to "B", in direct analogy with the definition of formula_136
If "E" happens to be path-connected and simply connected, this sequence reduces to an isomorphism
which generalizes the above fact about the universal covering (which amounts to the case where the fiber "F" is also discrete). If instead "F" happens to be connected and simply connected, it reduces to an isomorphism
What is more, the sequence can be continued at the left with the higher homotopy groups formula_139 of the three spaces, which gives some access to computing such groups in the same vein.
Classical Lie groups.
Such fiber sequences can be used to inductively compute fundamental groups of compact classical Lie groups such as the special unitary group formula_140 with formula_141 This group acts transitively on the unit sphere formula_142 inside formula_143 The stabilizer of a point in the sphere is isomorphic to formula_144 It then can be shown that this yields a fiber sequence
Since formula_146 the sphere formula_142 has dimension at least 3, which implies 
The long exact sequence then shows an isomorphism
Since formula_150 is a single point, so that formula_151 is trivial, this shows that formula_152 is simply connected for all formula_153
The fundamental group of noncompact Lie groups can be reduced to the compact case, since such a group is homotopic to its maximal compact subgroup. These methods give the following results:
A second method of computing fundamental groups applies to all connected compact Lie groups and uses the machinery of the maximal torus and the associated root system. Specifically, let formula_154 be a maximal torus in a connected compact Lie group formula_155 and let formula_156 be the Lie algebra of formula_157 The exponential map
is a fibration and therefore its kernel formula_159 identifies with formula_160 The map
can be shown to be surjective with kernel given by the set "I" of integer linear combination of coroots. This leads to the computation 
This method shows, for example, that any connected compact Lie group for which the associated root system is of type formula_163 is simply connected. Thus, there is (up to isomorphism) only one connected compact Lie group having Lie algebra of type formula_163; this group is simply connected and has trivial center.
Edge-path group of a simplicial complex.
When the topological space is homeomorphic to a simplicial complex, its fundamental group can be described explicitly in terms of generators and relations.
If "X" is a connected simplicial complex, an "edge-path" in "X" is defined to be a chain of vertices connected by edges in "X". Two edge-paths are said to be "edge-equivalent" if one can be obtained from the other by successively switching between an edge and the two opposite edges of a triangle in "X". If "v" is a fixed vertex in "X", an "edge-loop" at "v" is an edge-path starting and ending at "v". The edge-path group "E"("X", "v") is defined to be the set of edge-equivalence classes of edge-loops at "v", with product and inverse defined by concatenation and reversal of edge-loops.
The edge-path group is naturally isomorphic to π1(|"X"|, "v"), the fundamental group of the geometric realisation |"X"| of "X". Since it depends only on the 2-skeleton "X"2 of "X" (that is, the vertices, edges, and triangles of "X"), the groups π1(|"X"|,"v") and π1(|"X"2|, "v") are isomorphic.
The edge-path group can be described explicitly in terms of generators and relations. If "T" is a maximal spanning tree in the 1-skeleton of "X", then "E"("X", "v") is canonically isomorphic to the group with generators (the oriented edge-paths of "X" not occurring in "T") and relations (the edge-equivalences corresponding to triangles in "X"). A similar result holds if "T" is replaced by any simply connected—in particular contractible—subcomplex of "X". This often gives a practical way of computing fundamental groups and can be used to show that every finitely presented group arises as the fundamental group of a finite simplicial complex. It is also one of the classical methods used for topological surfaces, which are classified by their fundamental groups.
The "universal covering space" of a finite connected simplicial complex "X" can also be described directly as a simplicial complex using edge-paths. Its vertices are pairs ("w",γ) where "w" is a vertex of "X" and γ is an edge-equivalence class of paths from "v" to "w". The "k"-simplices containing ("w",γ) correspond naturally to the "k"-simplices containing "w". Each new vertex "u" of the "k"-simplex gives an edge "wu" and hence, by concatenation, a new path γ"u" from "v" to "u". The points ("w",γ) and ("u", γ"u") are the vertices of the "transported" simplex in the universal covering space. The edge-path group acts naturally by concatenation, preserving the simplicial structure, and the quotient space is just "X".
It is well known that this method can also be used to compute the fundamental group of an arbitrary topological space. This was doubtless known to Eduard Čech and Jean Leray and explicitly appeared as a remark in a paper by André Weil; various other authors such as Lorenzo Calabi, Wu Wen-tsün, and Nodar Berikashvili have also published proofs. In the simplest case of a compact space "X" with a finite open covering in which all non-empty finite intersections of open sets in the covering are contractible, the fundamental group can be identified with the edge-path group of the simplicial complex corresponding to the nerve of the covering.
Related concepts.
Higher homotopy groups.
Roughly speaking, the fundamental group detects the 1-dimensional hole structure of a space, but not holes in higher dimensions such as for the 2-sphere. Such "higher-dimensional holes" can be detected using the higher homotopy groups formula_165, which are defined to consist of homotopy classes of (basepoint-preserving) maps from formula_123 to "X". For example, the Hurewicz theorem implies that the "n"-th homotopy group of the "n"-sphere is (for all formula_167) are
As was mentioned in the above computation of formula_169 of classical Lie groups, higher homotopy groups can be relevant even for computing fundamental groups.
Loop space.
The set of based loops (as is, i.e., not taken up to homotopy) in a pointed space "X", endowed with the compact open topology, is known as the loop space, denoted formula_170 The fundamental group of "X" is in bijection with the set of path components of its loop space:
Fundamental groupoid.
The "fundamental groupoid" is a variant of the fundamental group that is useful in situations where the choice of a base point formula_83 is undesirable. It is defined by first considering the category of paths in formula_173 i.e., continuous functions
where "r" is an arbitrary non-negative real number. Since the length "r" is variable in this approach, such paths can be concatenated as is (i.e., not up to homotopy) and therefore yield a category. Two such paths formula_175 with the same endpoints and length "r", resp. "r"' are considered equivalent if there exist real numbers formula_176 such that formula_177 and formula_178 are homotopic relative to their end points, where formula_179
The category of paths up to this equivalence relation is denoted formula_180 Each morphism in formula_181 is an isomorphism, with inverse given by the same path traversed in the opposite direction. Such a category is called a groupoid. It reproduces the fundamental group since
More generally, one can consider the fundamental groupoid on a set "A" of base points, chosen according to the geometry of the situation; for example, in the case of the circle, which can be represented as the union of two connected open sets whose intersection has two components, one can choose one base point in each component. The van Kampen theorem admits a version for fundamental groupoids which gives, for example, another way to compute the fundamental group(oid) of formula_183
Local systems.
Generally speaking, representations may serve to exhibit features of a group by its actions on other mathematical objects, often vector spaces. Representations of the fundamental group have a very geometric significance: any "local system" (i.e., a sheaf formula_184 on "X" with the property that locally in a sufficiently small neighborhood "U" of any point on "X", the restriction of "F" is a constant sheaf of the form formula_185) gives rise to the so-called monodromy representation, a representation of the fundamental group on an "n"-dimensional formula_186-vector space. Conversely, any such representation on a path-connected space "X" arises in this manner. This equivalence of categories between representations of formula_48 and local systems is used, for example, in the study of differential equations, such as the Knizhnik–Zamolodchikov equations.
Étale fundamental group.
In algebraic geometry, the so-called étale fundamental group is used as a replacement for the fundamental group. Since the Zariski topology on an algebraic variety or scheme "X" is much coarser than, say, the topology of open subsets in formula_51 it is no longer meaningful to consider continuous maps from an interval to "X". Instead, the approach developed by Grothendieck consists in constructing formula_189 by considering all finite étale covers of "X". These serve as an algebro-geometric analogue of coverings with finite fibers.
This yields a theory applicable in situation where no great generality classical topological intuition whatsoever is available, for example for varieties defined over a finite field. Also, the étale fundamental group of a field is its (absolute) Galois group. On the other hand, for smooth varieties "X" over the complex numbers, the étale fundamental group retains much of the information inherent in the classical fundamental group: the former is the profinite completion of the latter.
Fundamental group of algebraic groups.
The fundamental group of a root system is defined, in analogy to the computation for Lie groups. This allows to define and use the fundamental group of a semisimple linear algebraic group "G", which is a useful basic tool in the classification of linear algebraic groups.
Fundamental group of simplicial sets.
The homotopy relation between 1-simplices of a simplicial set "X" is an equivalence relation if "X" is a Kan complex but not necessarily so in general. Thus, formula_169 of a Kan complex can be defined as the set of homotopy classes of 1-simplices. The fundamental group of an arbitrary simplicial set "X" are defined to be the homotopy group of its topological realization, formula_191 i.e., the topological space obtained by glueing topological simplices as prescribed by the simplicial set structure of "X".