Nonlinearity, Proper Actions and Equivariant Stable Cohomotopy

In this article we extend the classical definitions of equivariant cohomotopy theory to the setting of proper actions of Lie groups. We combine methods originally developed in the analysis of nonlinear differential equations, mainly in connection with Leray-Schauder theory, and on the other hand from developments of equivariant $K$-Theory by N.C. Phillips. We prove the correspondence with a previous construction of W. L\"uck by constructing an index. As an illustration of these methods, we introduce a Burnside ring defined in analytical terms. With this definition, we extend a weak version of the Segal Conjecture to a certain family of Lie groups and comment the relation to an invariant in Gauge Theory, due to Bauer and Furuta.