Homemath.OAarXiv:2605.29750

Elliptic Boundary Value Problems and Partial Group Actions

math.OAmath.APmath.FA2026-05v1license

Abstract

We consider a smooth compact manifold with boundary, MM, embedded in a smooth manifold of the same dimension on which an amenable group Γ\Gamma acts by isometries. We do not assume MM to be invariant under Γ\Gamma. This results in a {\em partial action} of Γ\Gamma on MM^\circ: For gΓg\in \Gamma we let Mg=g(M)MM^\circ_g = g(M^\circ)\cap M^\circ and obtain diffeomorphisms g:Mg1Mgg:M^\circ_{g^{-1}} \to M^\circ_g. We assume that any two images of M\partial M under Γ \Gamma either coincide or are disjoint and that only finitely many lie in MM. The spherical blow-up of these images of M\partial M in MM yields a manifold YY with boundary consisting of finitely many components. Moreover, YY inherits a partial action of~Γ\Gamma. We can then define the CC^*-algebra A=ΨΓ(Y,Y)\mathcal A=\overline{\Psi_\Gamma(Y,\partial Y)} of operators on L2(Y)L2(Y)L^2(Y)\oplus L^2(\partial Y), generated by the algebra Ψ(Y,Y)\Psi(Y,\partial Y) of operators of order and type zero in Boutet de Monvel's calculus on YY and partial isometries associated with the partial action. Denote by Σ=Ψ(Y,Y)/K\Sigma=\overline{\Psi(Y,\partial Y)}/\mathbb K its symbol space. If the partial action of Γ\Gamma on Prim(Σ)(\Sigma) is topologically free, we find a criterion for the Fredholm property of the operators in ΨΓ(Y,Y)\overline{\Psi_\Gamma(Y,\partial Y)}. Moreover, we obtain the classification of the elliptic elements in ΨΓ(Y,Y)\overline{\Psi_\Gamma(Y,\partial Y)} modulo stable homotopies: For A0=C(YY)Γ\mathcal A_0= C(Y\sqcup \partial Y)\rtimes\Gamma Ell(A0,A)K0(C0(TY)Γ)K0(C(Y)Γ).{\rm Ell}(\mathcal A_0,\mathcal A)\cong K_0(C_0(T^*Y^\circ)\rtimes\Gamma)\oplus K_0(C(\partial Y)\rtimes \Gamma). If Γ\Gamma is finitely generated and of polynomial growth, then the elements associated with the second summand do not contribute to the index.

Cite

@article{arxiv.2605.29750,
  title  = {Elliptic Boundary Value Problems and Partial Group Actions},
  author = {Eske Ewert and Anton Yu. Savin and Elmar Schrohe},
  journal= {arXiv preprint arXiv:2605.29750},
  year   = {2026}
}