The shifted wave equation on Damek--Ricci spaces and on homogeneous   trees
  We solve explicitly the shifted wave equation on Damek--Ricci spaces, using Asgeirsson's theorem and the inverse dual Abel transform. As an application, we investigate Huygens' principle. A similar analysis is carried out in the discrete setting of homogeneous trees. 
Jean-Philippe Anker (MAPMO), Pierre Martinot (IECN), Emmanuel Pedon   (LM-Reims), Alberto G. Setti
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