7 Construction of the assembly map
Let \(\Gamma \) be a countable discrete group for the KK-theoretic targets. The reduced crossed-product and cutoff prerequisites hold for arbitrary discrete groups unless countability is stated. Coefficient C\(^*\)-algebras may be nonunital; retain the separability hypotheses used by the existing KK quotients and products. Assembly spaces have continuous proper actions with the stated locally compact Hausdorff and second-countability hypotheses.
The constructions below give reduced descent and the coefficient-free assembly homomorphisms in both degrees under these hypotheses. Construction of these homomorphisms does not assert that they are isomorphisms.
7.1 Reduced crossed products and descent
For an action \(\alpha \) of \(\Gamma \) by complex star automorphisms of \(B\), form the actual finite-support coefficient star algebra \(C_c(\Gamma ,B)\). Its operations are
The sum is finite, and associativity and the star-algebra laws are proved for these operations.
Complete the finite vectors \(\Gamma \to B\) in the genuine right Hilbert \(B\)-module norm
The finite-vector coefficient-valued form is positive and definite; its null radical is proved zero. The canonical finite-vector image in the completed module \(\ell ^2(\Gamma ,B)\) is dense.
The actual adjointable regular operators satisfy
They obey the star and group laws, the covariance identity \(\pi _\alpha (\alpha _g(b))\lambda _g=\lambda _g\pi _\alpha (b)\), and \(\| \pi _\alpha (b)\| =\| b\| \), including for nonunital \(B\).
The coefficient-square order estimate bounds diagonal multiplication in the actual finite-vector form norm. Extend it and its proved adjoint to the completion. Translation is the unitary permutation of coordinates. The coordinate formulas give covariance on a dense set and hence on the completion. Applying \(\pi _\alpha (b)\) to the identity-coordinate vector with coefficient \(b^*\) gives the reverse norm inequality.
Define \(B\rtimes _{\alpha ,r}\Gamma \) as the operator-norm closed nonunital star subalgebra generated by the concrete \(\pi _\alpha (b)\lambda _g\) in \(\mathcal L_B(\ell ^2(\Gamma ,B))\). It is a C\(^*\)-algebra. The coefficient inclusion \(B\to B\rtimes _{\alpha ,r}\Gamma \) is isometric. Finite twisted convolution integrates by \(f\mapsto \sum _g\pi _\alpha (f(g))\lambda _g\) to a star homomorphism with dense range in this actual algebra.
The regular integrated representation is faithful on the finite-support algebra. Its operator norm is therefore a norm, and satisfies \(\| f^*f\| _r=\| f\| _r^2\). The reduced algebra is the closure of the actual finite generator span. If \(\Gamma \) is countable and \(B\) is separable, the constructed reduced crossed product is separable.
Evaluation of the integrated operator on an identity-coordinate vector recovers \(\alpha _{k^{-1}}(f(k))b\) at coordinate \(k\). If the operator vanishes, take \(b=\alpha _{k^{-1}}(f(k))^*\) and use the C\(^*\)-identity to show that every coefficient vanishes. The norm and its C\(^*\)-identity come from the ambient adjointable operators. Continuous coefficient families indexed by countably many group elements have separable range; their complex span and its closure remain separable.
For the trivial scalar action, the completed regular coefficient module is unitarily identified with the existing \(\ell ^2(\Gamma )\). The unitary sends \(z\delta _g\) to the actual scalar delta vector and intertwines the regular operators. It induces the concrete star-algebra equivalence
taking the scalar coefficient identity to the existing reduced group-algebra unit.
The finite-vector map \(z\delta _g\mapsto z\delta _g\) preserves the coefficient inner product and therefore the norm. Both finite-vector images are dense, so it extends to a Hilbert-module unitary. Conjugation by this unitary sends every crossed-product generator to \(z\lambda _g\). The closed generator spans on the two sides coincide under this equivalence and its inverse, giving the stated restricted star-algebra equivalence.
A compatible equivariant Hilbert \(B\)-module action \(U\) induces actual conjugation actions on \(\mathcal L_B(E)\) and \(\mathcal K_B(E)\). The compact inclusion intertwines them, and \(g\cdot \theta _{x,y}=\theta _{U_gx,U_gy}\).
Conjugation by \(U_g\), with inverse conjugation by \(U_{g^{-1}}\), is the constructed star-algebra automorphism of adjointable maps. The proved rank-one transport formula preserves finite-rank spans and their norm closure. Restrict the automorphism and its inverse to that compact ideal; the group laws and the equivariance of inclusion follow from the actual conjugation formulas.
For \(C=B\rtimes _r\Gamma \) and a complete Hilbert \(B\)-module \(E\), construct the completed interior module \(E\otimes _B C\) using the actual coefficient inclusion \(B\to C\). Elementary vectors obey the balancing relation and the inner-product formula
Every adjointable \(T\) acts by \(T\otimes 1\), with its adjoint and norm bound proved; compact module operators remain compact under this coefficient extension. If \(\Gamma \) is countable, \(B\) is separable and \(E\) is countably generated, the constructed module is countably generated. The integrated source action and quotient homomorphisms are separate statements below.
7.2 Integrated source action and genuine quotient descent
For an equivariant Hilbert \(B\)-module \(E\) and a possibly degenerate equivariant star representation of \(A\), finite source integration extends to the actual star homomorphism
It agrees with finite integration on generators and is bounded by the concrete reduced regular norm.
Embed the amplified descended module isometrically in the actual indexed regular module and intertwine finite integration with its regular coefficient action. For a degenerate source representation, localized approximate-unit vectors lie in the embedded range and suffice for the norm estimate; no surjectivity onto the whole regular module is assumed. Faithful tensor amplification detects the operator norm. Extend the bounded finite star homomorphism through its actual dense image.
Descent preserves genuine whole-cycle isomorphisms and full even, odd and graded homotopies. An interval witness may use any Hilbert module over \(C([0,1],B)\), rather than only an operator path on a fixed module.
Extend the actual interval module by the pointwise coefficient representation into \(C([0,1],B\rtimes _r\Gamma )\). The balanced diagonal action integrates globally. Its reduced bound follows from actual fibre norms and the supremum norm of the coefficient-valued inner product. The fibre/interior-tensor unitary intertwines the whole source representation, grading and operator. Its endpoint isomorphisms give the full target witness. Transport the original localized compact defects through actual tensor compactness and norm closure.
The actual descended cycles induce additive homomorphisms on the existing even and odd homotopy quotients. Their representative values are the constructed descended cycles. The ordinary target group action is \(\mathrm{Unit}\), and the coefficient and module carriers retain their actual enlarged universes.
The actual full-homotopy transport descends through the existing quotient relations. Completed tensor unitaries identify descent of direct sums with direct sums of descents, proving additivity. The actual opposite-cycle isomorphisms agree with negatives; zero follows by target-group cancellation. No new or weakened equivalence relation is introduced.
There is an actual graded Hilbert-module unitary
Transport of an actual original product operator satisfies the genuine descended cycle, connection and localized positivity conditions.
The comparison preserves balancing and the full coefficient-valued inner product, extends through completion, and has dense range by actual coefficient approximate units. Its pure-tensor formula intertwines the whole source action. The creation-map calculation proves all four connection compactness laws, including the adjoint companions, and extends by continuity. For positivity, transport the original positive-modulo-compacts condition and compress by arbitrary completed crossed source elements. A represented coefficient approximate unit converges on these products. Closedness of positivity modulo module compacts gives the full quadratic condition, including cross terms. Apply the existing product criterion and uniqueness theorem.
For all four existing native parity branches,
The group and coefficient universes are independent. This native comparison uses the common original module universe \(W=\max (u,u_\Gamma ,u_A,u_B,u_C)\); the target acting group remains \(\mathrm{Unit}\) at universe zero.
Compare actual graded product representatives using the tensor product criterion. Completed whole-cycle unitaries identify descent with the native even/graded and odd coordinate, signed-source and external-even Clifford pictures. The actual crossed pair star equivalence transports coefficient and source gradings. Existing injective bridges give the even-even, even-odd and odd-even laws. The odd-odd law uses the already genuine Pauli row/column Morita reduction to the coordinate/signed-source graded product. Thus the comparison uses the existing native products and signs; its conclusion is not an input.
Construct reduced descent on both actual quotient groups,
The representative formula uses the completed coefficient-extension module, the actual integrated reduced source action and \(F\otimes 1\). Prove compatibility with all four existing native Kasparov products in the common carrier universe stated above, and identify \(\mathbb C\rtimes _r\Gamma \cong C_r^*(\Gamma )\) by the actual scalar regular-module unitary.
7.3 Cutoff projections
A continuous proper action of a discrete group on a locally compact Hausdorff space with compact orbit quotient admits a nonnegative \(c\in C_c(X,\mathbb R)\) with \(\sum _g c(g^{-1}x)^2=1\). The sum has finite support at every point, and its topological sum has the same value. For any two compactly supported continuous functions \(c,d\), the coefficient family \(g\mapsto (x\mapsto c(x)d(g^{-1}x))\) has finite group support.
Every actual closed cocompact piece of a locally compact Hausdorff proper ambient action inherits properness and admits such a cutoff. Countability is not needed for these assertions.
Compact orbit representatives admit a nonnegative compactly supported seed \(f\) positive somewhere on every orbit. Properness makes its translated supports locally finite, so the invariant orbitwise square sum is continuous, finite and positive. Divide \(f\) by its square root. The resulting finite normalization identity also gives the convergent topological sum. Compact transporters bound the global support of every mixed coefficient. On a closed invariant piece, compact transporters are contained in those of the ambient proper action; apply the same construction to its compact intrinsic orbit quotient.
For normalized cutoffs \(c,d\), the exact mixed coefficient \(v_{c,d}(g)(x)=c(x)d(g^{-1}x)\) defines a finite-support element of \(C_c(\Gamma ,C_0(X))\). The actual twisted convolution and involution give \(v_{c,d}^*=v_{d,c}\) and \(v_{c,d}v_{d,e}=v_{c,e}\). Consequently \(p_c=v_{c,c}\) is a self-adjoint idempotent, and the mixed element implements Murray–von Neumann equivalence of \(p_c\) and \(p_d\).
Global finite support bundles the mixed coefficients in the actual twisted coefficient algebra. Inversion and conjugation exchange the two cutoffs. Expanding the coefficient of a product factors out \(c(x)e(g^{-1}x)\) and leaves the normalized sum \(\sum _t d(t^{-1}x)^2=1\). Thus the mixed elements multiply as stated. Taking equal cutoffs proves the projection identities, and exchanging the two cutoffs gives both partial-isometry equations.
An actual projection in a possibly nonunital C\(^*\)-algebra \(A\) defines its singleton-matrix projection class in the forced unitization. The scalar quotient of that matrix is zero, so the class belongs to the existing scalar-kernel group \(K_0(A)\). An actual partial isometry between two projections proves equality of these classes.
For a locally compact Hausdorff proper cocompact \(\Gamma \)-space, choose \(c\in C_c(X,[0,\infty ))\) with \(\sum _g c(g^{-1}x)^2=1\) and set \(p_c(g)(x)=c(x)c(g^{-1}x)\). Such a cutoff exists, \(p_c\) is a projection in \(C_0(X)\rtimes _r\Gamma \), and its K-class is independent of \(c\). The class is in the actual nonunital operator \(K_0\) group of the constructed reduced crossed product. Its independence follows from the explicit mixed-cutoff partial isometry in that completed algebra.
7.4 Naturality for actual coefficient and cutoff maps
An actual equivariant coefficient star homomorphism induces the actual reduced crossed-product map. The constructed parity descent commutes with the existing native source-pullback homomorphism used in Chapter 6.
Prove the finite coefficient-pushforward reduced norm bound and extend through concrete regular closures. On the actual same descent module, the two completed source representations agree, giving an identity-module whole-cycle isomorphism. The native homomorphism-cycle product bridge identifies its representatives with source precomposition. Passing through the genuine quotients proves naturality without assuming the desired compatibility.
The actual cutoff \(K_0\) class is natural under coefficient restrictions between cocompact pieces and under proper equivariant pullback, in the existing nonunital operator \(K_0\) groups of the actual reduced crossed products.
Properness preserves compact support of a pulled-back cutoff and equivariance preserves its normalized orbitwise sum. Compute the finite algebraic projection coefficients, pass through the actual reduced crossed homomorphism, and apply nonunital projection \(K_0\) naturality. The mixed-cutoff partial isometry removes the particular normalized cutoff choice.
7.5 Assembly on a piece
For an actual closed cocompact piece \(S\) of the indicated proper space, construct the additive homomorphism
Here \(c\) is an actual normalized cutoff on \(S\). Its genuine nonunital operator \(K_0\) class is viewed in scalar KK by the inverse established comparison. The map \(\chi _i\) uses the established scalar KK-to-operator-K comparison and actual scalar crossed-product identification. The even and odd codomains are exactly the existing \(\mathrm{ReducedK0}(\Gamma )\) and \(\mathrm{ReducedK1}(\Gamma )\) groups. The homomorphism is independent of the choice of cutoff.
7.6 Transitions and the actual colimit
Actual piece assembly homomorphisms commute with the existing Chapter 6 transitions and are independent of cutoff choice. They preserve assembly values under the actual bounded piece maps induced by ambient equivariant maps.
Use descent naturality for the exact native source-pullback product defining the transition. Move the actual cutoff class through reduced coefficient restriction by cutoff naturality, and apply the existing native product functoriality. For a bounded piece map, Chapter 6 supplies the genuine proper restriction. Global properness of the ambient equivariant map is not assumed.
Apply the actual Chapter 6 directed additive-group colimit to the concrete compatible piece family to construct
Its restriction along each actual piece inclusion is exactly \(\mu _{S,i}\), and the colimit universal property determines it uniquely. On the actual universal proper telescope and established Chapter 6 canonical topological group obtain
Both parity endpoints have the established analytic reduced K-group codomains. No arbitrary compatible family or desired compatibility witness is an additional input to these constructions.
Let \(\varphi :J\to \mathcal C_\Gamma (X)\) be a monotone map from a nonempty directed preorder to the actual cocompact pieces. The restricted assembly is the colimit of the same actual piece family. If
the established cofinal indexing equivalence preserves its assembly value. The comparison between two such genuinely cofinal systems does likewise.
Evaluate the restricted colimit map on a piece representative. The actual Chapter 6 reindex map sends it to the same piece of the full diagram, so the piece formula proves the assembly equality. Identifying the colimits retains the essential upper-bound cofinality hypothesis. For two cofinal systems, compose the first equivalence with the inverse of the second and cancel by the actual additive-equivalence laws. Mere pointwise coverage of the ambient space cannot replace cofinality.
The actual piece assembly homomorphisms commute with Chapter 6 transitions and bounded piece maps. They are independent of cutoff choice and compatible with the actual genuinely cofinal indexing equivalences. The resulting colimit assembly homomorphisms commute with actual ambient equivariant pushforwards, the established homotopy equivalences and genuine universal proper-model comparison.
Piece transition and bounded-map compatibility use the actual native source-pullback naturality and cutoff projection naturality. Every colimit class has an actual piece representative, so the piece identities give pushforward and homotopy-equivalence compatibility. The explicit cofinal index comparisons retain their essential upper-bound hypotheses. Apply the established fixed-point CW uniqueness equivalence for universal proper models; no arbitrary model equivalence is assumed.
This chapter concerns construction of the homomorphism. It does not prove injectivity, surjectivity, or the general Baum–Connes conjecture.
Reference for this route: S. Echterhoff, Bivariant KK-Theory and the Baum–Connes conjecture, Section 4.2, https://arxiv.org/abs/1703.10912.