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van Kampen theorem

Contents

Idea

The Seifert-van Kampen theorem is a classical theorem in algebraic topology which computes the fundamental group of a topological space in terms of a decomposition into open subsets.

It is most naturally expressed by saying that the fundamental groupoid functor preserves certain colimits. Here there is a bifurcation in possible generalizations, however. The colimits of spaces we consider (covers by open subsets) are both strict colimits and homotopy colimits. But we can ask the resulting colimit of groupoids to be either a homotopy colimit (i.e. a 2-colimit) or a strict colimit. The first is more natural for formal work; the second is more useful for computation. Accordingly, we will speak of homotopy van Kampen theorems and strict van Kampen theorems.

Statement

For fundamental groups

The most classical version of the Seifert-van Kampen theorem for the fundamental group is the following.

Theorem

Let XX be a topological space covered by open subsets U,VXU,V \subset X such that UVU \cap V is path connected. Then for every choice of basepoint xUVx \in U \cap V, the diagram of homotopy groups

π 1(UV,x) π 1(U,x) π 1(V,x) π 1(X,x) \array{ \pi_1(U \cap V, x) &\to& \pi_1(U,x) \\ \downarrow && \downarrow \\ \pi_1(V,x) &\to& \pi_1(X,x) }

is a pushout square in Grp.

From groups to groupoids

We can regard the classical theorem as being a statement about fundamental groupoids as follows. Recall that any group GG can be considered as a 1-object groupoid BG\mathbf{B}G by delooping. It is easy to see that the functor B\mathbf{B} takes colimits of groups to strict colimits of groupoids. Moreover, a strict pushout of one-object groupoids is also a 2-pushout, because the maps we are pushing out are bijective on objects, hence cofibrations in the canonical model structure on GpdGpd. Thus, under the hypotheses of the classical van Kampen theorem, we have a square

Bπ 1(UV,x) Bπ 1(U,x) Bπ 1(V,x) Bπ 1(X,x) \array{ \mathbf{B}\pi_1(U \cap V, x) &\to& \mathbf{B}\pi_1(U,x) \\ \downarrow && \downarrow \\ \mathbf{B}\pi_1(V,x) &\to& \mathbf{B}\pi_1(X,x) }

which is both a strict pushout and a 2-pushout in GpdGpd.

Now let XX' be the path component? of XX which contains UVU\cap V (there is exactly one such, because UVU\cap V is path-connected), and similarly UU' and VV'. Then the fundamental groupoid Π 1(X)\Pi_1(X') is equivalent to Bπ 1(X,x)\mathbf{B}\pi_1(X,x), and similarly in the other cases. Since 2-pushouts are invariant under equivalence of groupoids, we also have a 2-pushout square

Π 1(UV) Π 1(U) Π 1(V) Π 1(X) \array{ \Pi_1(U \cap V) &\to& \Pi_1(U') \\ \downarrow && \downarrow \\ \Pi_1(V') &\to& \Pi_1(X') }

This is an instance of what we referred to above as a homotopy van Kampen theorem. But we can easily make it a strict van Kampen theorem as well.

Note that the functors Π 1(UV)Π 1(U)\Pi_1(U\cap V) \to \Pi_1(U') and Π 1(UV)Π 1(U)\Pi_1(U\cap V) \to \Pi_1(U') are injective on objects, hence cofibrations in the canonical model structure, and so their strict pushout is also a 2-pushout. This strict pushout is therefore equivalent to Π 1(X)\Pi_1(X'). (It need not be isomorphic to it, but this can be remedied by defining the fundamental groupoid on a set of specified basepoints as below.)

We now consider more general versions of the van Kampen theorem which do not require UVU\cap V to be path-connected, and which compute all of Π 1(X)\Pi_1(X) rather than just Π 1(X)\Pi_1(X').

Strict van Kampen theorem for groupoids

Let us define Π 1(X,X 0)\Pi_1(X,X_0) of a space XX and a subset X 0XX_0 \subseteq X to be the full subgroupoid of Π 1(X)\Pi_1(X) on the set X 0X_0. More generally, if UU is given as a subspace of XX and X 0XX_0 \subseteq X, we write Π 1(U,X 0)\Pi_1(U,X_0) for Π 1(U,X 0U)\Pi_1(U,X_0\cap U).

If X *=(X,X 0)X_* =(X,X_0) is a pair consisting of a space XX and a set X 0X_0 of base points, we say X *X_* is connected if X 0X_0 meets each path component of XX. This is clearly a necessary and sufficient condition for Π 1(X,X 0)\Pi_1(X,X_0) to be equivalent to Π 1(X)\Pi_1(X).

Now let {U iX}\{U^i \to X\} for iIi \in I be an open cover of XX. If d=(i,j)I 2d=(i,j) \in I^2 we write U dU^d for U iU jU^i \cap U^j, and similarly for nn-fold intersections. For any nn-tuple eI ne\in I^n, let U * eU^e_* be the pair (U e,X 0U e)(U^e,X_0 \cap U^e). We then have a coequaliser diagram of pairs of spaces, where a,b,ca,b,c are determined by inclusions:

dI 2U * dba iIU * icX *. \bigsqcup_{d \in I^2} U^d_* \stackrel{\overset{a}{\to}}{\underset{b}{\to}} \bigsqcup _{i \in I} U^i_* \xrightarrow{c} X_* \,.
Theorem

If the pairs of spaces U * fU^f_* are connected for all 1-, 2-, or 3-fold intersections U * fU^f_* of the pairs U * iU^i_*, then

  1. (Conn) The pair X *X_* is connected; and

  2. (Iso) The fundamental groupoid functor Π 1\Pi_1 takes the above coequaliser diagram of pairs of spaces to a (strict) coequaliser diagram of groupoids.

The limit “3” on the connectivity hypothesis may be analyzed as “1+21+2”, where 11 is the categorical dimension of the fundamental groupoids we are considering and 22 is a universal constant. This becomes evident when generalizing to higher homotopy van Kampen theorems.

In one proof of the theorem (due to Brown and Razak, see below), the number 3 arises from the Lebesgue covering dimension of 2\mathbb{R}^2. This proof verifies the universal property for a coequaliser, using basic techniques: subdivide a path; deform a subdivision so that it is product of paths joining points of X 0X_0; subdivide a homotopy rel end points, and deform this subdivision so that all subpaths join points of X 0X_0; any composition of commutative squares in a groupoid is commutative.

In another, more categorical proof explained in the next section, the number 33 arises from the fact that 2-colimits can be computed using coproducts and descent objects of 2-truncated simplicial objects.

Note that the theorem about groupoids does not reduce to a theorem about groups. For example, XX may be a connected space which is the union of two open sets each with many components and whose intersection has many path components. Such examples arise commonly from applications to combinatorial group theory; consider for example a covering space of a wedge of spaces. Or the connected XX may be the union of 23 open sets whose three fold intersections have 123 path components. In each case the fundamental group one might want to calculate is in the middle of this complicated combinatorial situation, but at least the theorem has turned a topological problem into a group theory and combinatorial problem, and the remarkable fact is that the fundamental groups are completely determined by the theorem. This is an “anomaly” for traditional algebraic topology, where invariants relating adjacent dimensions may be determined by exact sequences which do not give complete information. The reason for the success seems to be that groupoids have structure in dimensions 0 and 1, and so can model the gluing of spaces.

This suggests that higher homotopy van Kampen theorems could give new kinds of homotopical information, i.e. colimit theorems for higher homotopy invariants, which has proved to be so.

Homotopy van Kampen theorem for groupoids

From a 2-categorical point of view, the strict van Kampen theorem above can be decomposed into the following parts:

  1. When XX is the union of a family of open sets, then it is the homotopy colimit of a diagram involving these open sets.

  2. The fundamental groupoid functor preserves homotopy colimits (i.e. takes them to homotopy colimits, or equivalently 2-colimits, of groupoids).

  3. Certain 2-colimits of groupoids can be computed over “2-truncated” subdiagrams.

  4. In certain situations, strict colimits of groupoids are also 2-colimits.

We can then prove the theorem by proving each of these three parts separately. This is essentially accomplished in Farjoun.

Theorem

If a space XX is the union of a family of open sets (U i) iI(U^i)_{i\in I}, then we have a weak equivalence

hocolimC (U)X, hocolim C_\bullet(U) \xrightarrow{\sim} X,

where C (U)C_\bullet(U) is the Cech cover of XX, a simplicial space defined by

C n(U)= JI,|J|=n iJU i.C_n(U) = \coprod_{J\subseteq I,\, |J|=n} \bigcap_{i\in J} U^i.
Proof

See Dugger-Isaksen.

Theorem

Suppose II is a small category and (X i) iI(X^i)_{i\in I} is a diagram of spaces. Then we have an equivalence of groupoids

Π 1(hocolim iX i)hocolim iΠ 1(X i). \Pi_1(\hocolim_i X^i) \simeq \hocolim_i \Pi_1(X^i).
Proof

The functor Π 1\Pi_1 is, up to equivalence, the composite of itself with the 1st Postnikov approximation? functor P 1:TopTopP_1 : Top \to Top. We can regard P 1P_1 as a Bousfield localization functor for TopTop (for localizing at the map S 2*S^2 \to \ast), so it preserves homotopy colimits. And Π 1\Pi_1 is part of a Quillen equivalence between GpdGpd and the 1-localized model structure on TopTop, so it also preserves homotopy colimits.

Theorem

If XX is the union of (U i) iI(U^i)_{i\in I}, and X 0XX_0\subseteq X is a set of basepoints which intersects each 1-, 2-, and 3-fold intersection of opens U iU^i, then Π 1(X,X 0)\Pi_1(X,X_0) is equivalent to the 2-colimit (i.e. codescent object?) of the following diagram

i,j,kΠ 1(U ijk,X 0) i,jΠ 1(U ij,X 0) iΠ 1(U i,X 0). \coprod_{i,j,k} \Pi_1(U^{i j k},X_0) \;\underoverset{\to}{\to}{\to}\; \coprod_{i,j} \Pi_1(U^{i j},X_0) \;\rightrightarrows\; \coprod_i \Pi_1(U^i,X_0).
Proof

If we neglect the X 0X_0‘s, then what we have above is the 2-truncated part of the homotopy colimit of groupoids obtained by applying Theorem 2 to Theorem 1. Since homotopy colimits of simplicial objects in GpdGpd are equivalent to codescent objects of their 2-truncations (essentially because groupoids are 1-groupoids), this has an equivalent homotopy colimit. Finally, the assumption on X 0X_0 ensures that restricting to it as a set of basepoints does not change the equivalence type of any of the groupoids in the 2-truncated diagram.

Theorem

Under the hypotheses of Theorem 3, we have that Π 1(X,X 0)\Pi_1(X,X_0) is isomorphic to the strict coequalizer of

i,jΠ 1(U ij,X 0) iΠ 1(U i,X 0). \coprod_{i,j} \Pi_1(U^{i j},X_0) \;\rightrightarrows\; \coprod_i \Pi_1(U^i,X_0).
Proof

Let A=P(I)A = P(I) be the power set of the indexing set II, regarded as a poset, and consider the functor F:AGpdF:A\to Gpd defined by F(J)=Π 1( iJU i,X 0)F(J) = \Pi_1(\bigcap_{i\in J} U^i, X_0). We can verify that the 2-colimit of FF is equivalent to the codescent object in Theorem 3, for essentially the same reasons cited there. (Formally, one can say that the functor P 3(I)AP_{\le 3}(I) \to A is “2-final”.)

However, FF is cofibrant in the projective model structure on Gpd AGpd^A, which is to say that the functor AFGpdobSetA\xrightarrow{F} Gpd \xrightarrow{ob} Set is free (relative to the forgetful functor Set ASet ob(A)Set^A \to Set^{ob(A)}): its generators are the points xF(J)x\in F(J) such that J={iI|xU i}J = \{ i\in I \,|\, x\in U^i \}. Thus, the 2-colimit of FF is equivalent to its strict colimit, which in turn is easily seen to be the coequalizer shown above.

This shows that Π 1(X,X 0)\Pi_1(X,X_0) is equivalent to the strict coequalizer shown above. But this equivalence is clearly also bijective on objects, so it is an isomorphism of groupoids.

Examples

Let X=S 1X=S^1, U=S 1{N}U= S^1 \setminus \{N\}, and U=S 1{S}U= S^1 \setminus \{S\}, where NN and SS are antipodal points. Then UV(0,1)U\cong V\cong (0,1) are contractible, while UV(0,1)(0,1)U\cap V \cong (0,1) \sqcup (0,1). Thus the homotopy van Kampen theorem gives a 2-pushout of groupoids that is equivalent to

** * * Π 1(S 1) \array{ \ast\sqcup \ast & \to & \ast \\ \downarrow && \downarrow \\ \ast & \to & \Pi_1(S^1) }

It’s not hard to compute that this 2-pushout must be B\mathbf{B}\mathbb{Z}. But we can also obtain this directly from the strict van Kampen theorem with a set X 0={u,v}X_0 = \{u,v\} of two basepoints with uUu\in U and vVv\in V, which yields a strict pushout of groupoids

** Π 1(S 1,X 0) \array{ \ast\sqcup \ast & \to & \mathcal{I} \\ \downarrow && \downarrow \\ \mathcal{I} & \to & \Pi_1(S^1,X_0) }

where \mathcal{I} is the invertible interval category, which is (literally) Π 1(U,X 0)\Pi_1(U,X_0) and Π 1(V,X 0)\Pi_1(V,X_0).

Note that the version of the theorem for fundamental groups does not apply, since UVU\cap V is not path-connected.

Generalizations

References

The following paper gives the general theorem for the fundamental groupoid on a set of base points:

  • Ronnie Brown and A. Razak, A van Kampen theorem for unions of non-connected spaces, Archiv. Math. 42 (1984) 85-88.

Applications to 11-dimensional homotopy theory are given in:

Ronnie Brown Topology and Groupoids, Booksurge (2006). ([web page] http://pages.bangor.ac.uk/~mas010/topgpds.html)

The proof for pushouts is discussed in some detail in

as a background to the following chapters on versions for crossed modules and crossed complexes, i.e. for higher dimensional versions.

The proof via homotopy colimits is roughly taken from

Other references cited above:

Revised on August 18, 2013 17:43:27 by Ronnie Brown (81.129.231.119)