Linear Boltzmann equation (${\partial }_{t}f+v\cdot {\nabla }_{x}f=K_{t}f-af$)

A linear kinetic transport equation with velocity-integral gain $K_t$ and nonnegative loss coefficient $a$. Its characteristic integral formulation combines the damped free-transport evolution with the Boltzmann Volterra operator. A uniform square-integrable velocity kernel yields bounded gain on $L_{x,v}^2$ and a globally convergent Volterra series for the linear Boltzmann equation on every finite time interval.

Let $M\ge 0$ have velocity integral one, and $\rho (g)(x)=\int g(x,v)dv$. On phase-space $L^1$, the normalized relaxation collision operator is $B=P-I$, where $Pg=\rho (g)M$. It obeys $\Vert B\Vert \le 2$ because $\Vert Pg{\Vert }_1\le \Vert g{\Vert }_1$. It preserves mass at each spatial position. The corresponding linear Boltzmann equation combines this bounded collision operator with the free-transport semigroup.

For $E=L_{x,v}^1$, let $(\tau f)(t)={\int }_0^t{U}_{t-s}(P-I)f(s)ds$. The factorial bound for a Volterra iterate is $\Vert {\tau }^{n}f(t){\Vert }_1\le (2t{)}^n{\sup }_{s\le t}\Vert f(s){\Vert }_1/n!$. Thus the Neumann series converges in $C([0,T];E)$ for every finite $T$ and solves $f=U_{\cdot }{f}_0+\tau f$. The same estimate applied to a difference proves uniqueness among locally time-bounded integral solutions. The whole collision term remains inside this undamped integral, rather than absorbing loss into the free propagator.

For a nonnegative normalized velocity density $M$, $Pg(x,v)=M(v)\int g(x,w)dw$ is a positive norm-one projection on phase-space $L^1$. Normalization gives $P^2=P$. Its range consists of fields $a(x)M(v)$. This Banach-space projection need not be an orthogonal projection in unweighted $L^2$; no such assertion is needed for the $L^1$ collision estimates.

The factorial bound for a Volterra iterate gives convergence in the space of bounded strongly measurable $L^2$-valued functions on $[0,T]$. The sum solves $(I-\tau )f=F$, where $F=U_a(t,0)f_0$, and satisfies ${\sup }_{t\le T}\Vert f(t){\Vert }_2\le e^{CT}\Vert f_0{\Vert }_2$. Applying the same iterate bound to a difference $g=\tau g$ proves uniqueness on every finite interval.

Combine the damped free-transport evolution and the linear Boltzmann collision operator in a time-ordered Bochner integral. Its norm obeys $\Vert \tau f(t){\Vert }_2\le C{\int }_0^t\Vert f(s){\Vert }_{2}ds$. This reduces the characteristic integral equation to $(I-\tau )f=U_a(t,0)f_0$.

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.

The gain term integrates over the incoming velocity. If $C^2=\int {\sup }_{t,x}|k(t,x,v,v_{\ast }){|}^{2}dvd{v}_{\ast }<\infty$, the Hilbert-Schmidt kernel bound at each $x$ gives $\Vert K_{t}g{\Vert }_{L_{x,v}^2}\le C\Vert g{\Vert }_{L_{x,v}^2}$, uniformly in time. The existence proof uses boundedness; kernel positivity is not needed for that estimate.