Solution Representations for a Wave Equation with Weak Dissipation

We consider the Cauchy problem for the weakly dissipative wave equation $$\bx v+\frac\mu{1+t}v_t=0, \qquad x\in\R^n,\quad t\ge 0,$$ parameterized by $\mu>0$, and prove a representation theorem for its solution using the theory of special functions. This representation is used to obtain $L_p$--$L_q$ estimates for the solution and for the energy operator corresponding to this Cauchy problem. Especially for the $L_2$ energy estimate we determine the part of the phase sp which is responsible for the decay rate. It will be shown that the situation d strongly on the value of $\mu$ and that $\mu=2$ is critical.