Damped free-transport evolution ($U_a(t,s)$)

For nonnegative measurable $a$, define $(U_a(t,s)g)(x,v)=e^{-{\int }_s^{t}a(r,x-v(t-r),v)dr}g(x-v(t-s),v)$. The shear preserves Lebesgue measure and the multiplier lies in $[0,1]$, so this is a contraction on Lp spaces. Extended nonnegative integrals use $e^{-\infty }=0$. Characteristic local integrability gives the usual continuous trace at the starting time.