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This book uses the hypoelliptic Laplacian to evaluate semisimple orbital integrals in a formalism that unifies index theory and the trace formula. The hypoelliptic Laplacian is a family of operators that is supposed to interpolate between the ordinary Laplacian and the geodesic flow. It is essentially the weighted sum of a harmonic oscillator along the fiber of the tangent bundle, and of the generator of the geodesic flow. In this book, semisimple orbital integrals associated with the heat kernel of the Casimir operator are shown to be invariant under a suitable hypoelliptic deformation, which is constructed using the Dirac operator of Kostant. Their explicit evaluation is obtained by localization on geodesics in the symmetric space, in a formula closely related to the Atiyah-Bott fixed point formulas. Orbital integrals associated with the wave kernel are also computed. Estimates on the hypoelliptic heat kernel play a key role in the proofs, and are obtained by combining analytic, geometric, and probabilistic techniques. Analytic techniques emphasize the wavelike aspects of the hypoelliptic heat kernel, while geometrical considerations are needed to obtain proper control of the hypoelliptic heat kernel, especially in the localization process near the geodesics. Probabilistic techniques are especially relevant, because underlying the hypoelliptic deformation is a deformation of dynamical systems on the symmetric space, which interpolates between Brownian motion and the geodesic flow. The Malliavin calculus is used at critical stages of the proof.

Differential equations, Hypoelliptic. --- Laplacian operator. --- Definite integrals. --- Orbit method. --- Bianchi identity. --- Brownian motion. --- Casimir operator. --- Clifford algebras. --- Clifford variables. --- Dirac operator. --- Euclidean vector space. --- Feynman-Kac formula. --- Gaussian integral. --- Gaussian type estimates. --- Heisenberg algebras. --- Kostant. --- Leftschetz formula. --- Littlewood-Paley decomposition. --- Malliavin calculus. --- Pontryagin maximum principle. --- Selberg's trace formula. --- Sobolev spaces. --- Toponogov's theorem. --- Witten complex. --- action functional. --- complexification. --- conjugations. --- convergence. --- convexity. --- de Rham complex. --- displacement function. --- distance function. --- elliptic Laplacian. --- elliptic orbital integrals. --- fixed point formulas. --- flat bundle. --- general kernels. --- general orbital integrals. --- geodesic flow. --- geodesics. --- harmonic oscillator. --- heat kernel. --- heat kernels. --- heat operators. --- hypoelliptic Laplacian. --- hypoelliptic deformation. --- hypoelliptic heat kernel. --- hypoelliptic heat kernels. --- hypoelliptic operators. --- hypoelliptic orbital integrals. --- index formulas. --- index theory. --- infinite dimensional orbital integrals. --- keat kernels. --- local index theory. --- locally symmetric space. --- matrix part. --- model operator. --- nondegeneracy. --- orbifolds. --- orbital integrals. --- parallel transport trivialization. --- probabilistic construction. --- pseudodistances. --- quantitative estimates. --- quartic term. --- real vector space. --- refined estimates. --- rescaled heat kernel. --- resolvents. --- return map. --- rough estimates. --- scalar heat kernel. --- scalar heat kernels. --- scalar hypoelliptic Laplacian. --- scalar hypoelliptic heat kernels. --- scalar hypoelliptic operator. --- scalar part. --- semisimple orbital integrals. --- smooth kernels. --- standard elliptic heat kernel. --- supertraces. --- symmetric space. --- symplectic vector space. --- trace formula. --- unbounded operators. --- uniform bounds. --- uniform estimates. --- variational problems. --- vector bundles. --- wave equation. --- wave kernel. --- wave operator.