Vanishing corrections for the position in a linear model of FKPP fronts

Take the linearised FKPP equation $${\mathrm{\partial }}_{t}h={\mathrm{\partial }}_x^{2}h+h$$ with boundary condition $h(m(t),t)=0$. Depending on the behaviour of the initial condition $h_0(x)=h(x,0)$ we obtain the asymptotics - up to a $o(1)$ term $r(t)$ - of the absorbing boundary $m(t)$ such that $\omega(x):=\lim_t h(x+m(t) ,t)$ exists and is non-trivial. In particular, as in Bramson’s results for the non-linear FKPP equation, we recover the celebrated $-(3\mathrm{/}2)\log t$ correction for initial conditions decaying faster than $x^{\nu }{e}^{-x}$ for some $\nu <-2$. Furthermore, when we are in this regime, the main result of the present work is the identification (to first order) of the $r(t)$ term which ensures the fastest convergence to $\omega (x)$. When $h_0(x)$ decays faster than $x^\nu e^{-x}$forsome$\nu<-3$,weshowthat$r(t)$mustbechosentobe$-3\sqrt{\pi/t}$ which is precisely the term predicted heuristically by Ebert-van Saarloos in the non-linear case. When the initial condition decays as $x^{\nu }{e}^{-x}$ for some $\nu \in [-3,-2)$, we show that even though we are still in the regime where Bramson’s correction is $-(3\mathrm{/}2)\log t$, the Ebert-van Saarloos correction has to be modified. Similar results were recently obtained by Henderson using an analytical approach and only for compactly supported initial conditions.