Supersymmetric properties of one-dimensional Markov generators with the links to Markov-dualities and to shape-invariance-exact-solvability

Abstract

For diffusion process involving the force $F(x)$ and the diffusion coefficient $D(x)$, the continuity equation $\partial_t P_t(x)=- \partial_xJ_t(x)$ gives the dynamics of the probability $P_t( x)$ in terms of the current $J_t( x)=F(x)P_t(x)-D(x)\partial_x P_t(x)={\cal J}P_t( x)$ obtained from $P_t( x) $ via the application of the first-order differential current-operator ${\cal J}$. So the dynamics of the probability $P_t( x)$ is governed by the factorized Fokker-Planck generator ${\cal F}=-\partial_x{\cal J}$, while the dynamics of the current $J_t( x)$ is governed by its supersymmetric partner ${\hat {\cal F} }= - {\cal J}\partial_x$, so that their right and left eigenvectors are directly related using the two intertwining relations ${\cal J}{\cal F}=-{\cal J}\partial_x{\cal J}={\hat {\cal F}}{\cal J}$ and ${\cal F}\partial_x=-\partial_x{\cal J}\partial_x=\partial_x{\hat {\cal F} }$. We also describe the link with the factorization of the adjoint $ {\cal F}^{\dagger}=\frac{d}{dm(x)}\frac{d}{ds(x)} $ in terms of the scale function $s(x)$ and speed measure $m(x)$. We then analyze how the supersymmetric partner ${\hat {\cal F} } = - {\cal J} \partial_x$ can be re-interpreted in two ways: (1) as the adjoint ${\mathring {\cal F}}^{\dagger} ={\mathring {\cal J} }^{\dagger} \partial_x$ of the Fokker-Planck generator ${\mathring {\cal F}}=- \partial_x{\mathring {\cal J} }$ associated to the dual force ${\mathring F}(x)=-F(x)-D'(x)$, that unifies various known Markov dualities; (2) as the non-conserved Fokker-Planck generator ${\tilde {\cal F}}_{nc} = -\partial_x{\tilde {\cal J}}-{\tilde K }(x)$ involving the force ${\tilde F}(x)=F(x) +D'(x)$ and the killing rate ${\tilde K }(x)=-F'(x)-D''(x)$, with application to shape-invariance-solvability. Finally, we describe how all these ideas can be also applied to Markov jump processes with nearest-neighbors transition rates $w(x \pm 1,x)$.