In recent time, by working in a plane with the metric associated with wave equation (the Special Relativity non-definite quadratic form), a complete formalization of space-time trigonometry and a Cauchy-like integral formula have been obtained. In th
is paper the concept that the solution of a mathematical problem is simplified by using a mathematics with the symmetries of the problem, actuates us for studying the wave equation (in particular the initial values problem) in a plane where the geometry is the one generated by the wave equation itself. In this way, following a classical approach, we point out the well known differences with respect to Laplace equation notwithstanding their formal equivalence (partial differential equations of second order with constant coefficients) and also show that the same conditions stated for Laplace equation allow us to find a new solution. In particular taking as initial data for the wave equation an arbitrary function given on an arm of an equilateral hyperbola, a Poisson-like integral formula holds.
