# Jost function

In scattering theory, the Jost function is the Wronskian of the regular solution and the (irregular) Jost solution to the differential equation ${\displaystyle -\psi ''+V\psi =k^{2}\psi }$. It was introduced by Res Jost.

## Background

We are looking for solutions ${\displaystyle \psi (k,r)}$ to the radial Schrödinger equation in the case ${\displaystyle \ell =0}$,

${\displaystyle -\psi ''+V\psi =k^{2}\psi .}$

## Regular and irregular solutions

A regular solution ${\displaystyle \varphi (k,r)}$ is one that satisfies the boundary conditions,

{\displaystyle {\begin{aligned}\varphi (k,0)&=0\\\varphi _{r}'(k,0)&=1.\end{aligned}}}

If ${\displaystyle \int _{0}^{\infty }r|V(r)|<\infty }$, the solution is given as a Volterra integral equation,

${\displaystyle \varphi (k,r)=k^{-1}\sin(kr)+k^{-1}\int _{0}^{r}dr'\sin(k(r-r'))V(r')\varphi (k,r').}$

We have two irregular solutions (sometimes called Jost solutions) ${\displaystyle f_{\pm }}$ with asymptotic behavior ${\displaystyle f_{\pm }=e^{\pm ikr}+o(1)}$ as ${\displaystyle r\to \infty }$. They are given by the Volterra integral equation,

${\displaystyle f_{\pm }(k,r)=e^{\pm ikr}-k^{-1}\int _{r}^{\infty }dr'\sin(k(r-r'))V(r')f_{\pm }(k,r').}$

If ${\displaystyle k\neq 0}$, then ${\displaystyle f_{+},f_{-}}$ are linearly independent. Since they are solutions to a second order differential equation, every solution (in particular ${\displaystyle \varphi }$) can be written as a linear combination of them.

## Jost function definition

The Jost function is

${\displaystyle \omega (k):=W(f_{+},\varphi )\equiv \varphi _{r}'(k,r)f_{+}(k,r)-\varphi (k,r)f_{+,x}'(k,r)}$,

where W is the Wronskian. Since ${\displaystyle f_{+},\varphi }$ are both solutions to the same differential equation, the Wronskian is independent of r. So evaluating at ${\displaystyle r=0}$ and using the boundary conditions on ${\displaystyle \varphi }$ yields ${\displaystyle \omega (k)=f_{+}(k,0)}$.

## Applications

The Jost function can be used to construct Green's functions for

${\displaystyle \left[-{\frac {\partial ^{2}}{\partial r^{2}}}+V(r)-k^{2}\right]G=-\delta (r-r').}$

In fact,

${\displaystyle G^{+}(k;r,r')=-{\frac {\varphi (k,r\wedge r')f_{+}(k,r\vee r')}{\omega (k)}},}$

where ${\displaystyle r\wedge r'\equiv \min(r,r')}$ and ${\displaystyle r\vee r'\equiv \max(r,r')}$.

## References

• Roger G. Newton, Scattering Theory of Waves and Particles.
• D. R. Yafaev, Mathematical Scattering Theory.