The Method of Ascent and cos(sqrt(A2+b2))
Abstract
The fundamental solution for the wave equation in n variables is built from the simple one-dimensional formula, via an integral representation of the cosine of the sum of squares of self-adjoint operators. Representation formulas are given both in the commutative and non-commutative case. The formulas are illustrated for the wave equation as well as for the Klein-Gordon equati on. As examples for the non-commuting case, we discuss the harmonic oscillator and simple sum of squares hypoelliptic operators such as the Grushin operator and the Heisenberg Laplacian.
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