The explicit probability distribution of the sum of two telegraph processes

Abstract

We consider two independent Goldstein–Kac telegraph processes X1(t) and X2(t) on the real line ℝ, both developing with constant speed c > 0, that, at the initial time instant t = 0, simultaneously start from the origin 0 ∈ ℝ and whose evolutions are controlled by two independent homogeneous Poisson processes of the same rate λ > 0. Closed-form expressions for the transition density φ(x, t) and the probability distribution function Φ(x, t) = Pr {S(t) < x}, x ∈ ℝ, t > 0, of the sum S(t) = X1(t) + X2(t) of these processes at arbitrary time instant t > 0, are obtained. It is also proved that the shifted time derivative g(x, t) = (∂/∂t + 2λ)φ(x, t) satisfies the Goldstein–Kac telegraph equation with doubled parameters 2c and 2λ. From this fact it follows that φ(x, t) solves a third-order hyperbolic partial differential equation, but is not its fundamental solution. The general case is also discussed.