Abstract
We consider the propagation of wave fronts connecting unstable and stable uniform solutions to a discrete reaction–diffusion equation on a onedimensional integer lattice. The dependence of the wavespeed on the coupling strength \(\mu \) between lattice points and on a detuning parameter (a) appearing in a nonlinear forcing is investigated thoroughly. Via asymptotic and numerical studies, the speed both of ‘pulled’ fronts (whereby the wavespeed can be characterised by the linear behaviour at the leading edge of the wave) and of ‘pushed’ fronts (for which the nonlinear dynamics of the entire front determine the wavespeed) is investigated in detail. The asymptotic and numerical techniques employed complement each other in highlighting the transition between pushed and pulled fronts under variations of \(\mu \) and a.
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Notes
Due to the rescaling of j, the constant c in the remainder of this section corresponds to dividing that elsewhere in the paper by \(\sqrt{\mu }\).
We stress that, because \(x=j/\sqrt{\mu }\), the quantities z, S, c and \(\lambda \) henceforth are scaled differently from those in the previous section.
Provided—as in the case of the cubic nonlinearity on which we mainly focus—only one such transition occurs; a characterisation of the nonlinearities f(u; a) in such regards would be valuable.
We suppress the dependence of U on \(\mu \) in our farfield expressions.
Note that \(c^\dag (a_T,\mu )=c^*(\mu )\).
Here we assume \(a={O}(1)\); the behaviour differs significantly for \(a={O}(\varepsilon )\)—see Sect. 5.3 below.
Setting the coefficient of the exponential to unity in the initial condition requires appropriate choice of the \({O}\left( 1/\ln (1/\varepsilon ) \right) \) contribution to \(T_j(\varepsilon )\).
That the other terms from the central difference operator are negligible in (85) follows from the factors \(\mathrm{e}^{s}\) arising from the transformation from U to V.
Because \(\zeta \ll s\) in (78), this fully nonlinear region makes no leadingorder contribution to the integral in (109), though the contribution it does make is only logarithmically smaller: selfconsistency checks incorporating such correction terms have been undertaken to confirm that they do not lead to the expansions derived in this section becoming invalid in the regimes considered.
The notation \(\lambda _\) is that of Appendix 2, not that above; the two usages of \(\lambda _\pm \) are in correspondence, however.
It will be clear by now that we are, in the interests of brevity, including in a number of such expressions terms that may not be of the same order. We affirm, in line with the footnote before last, that the analysis is nevertheless not ad hoc: that the various terms that contribute to the final conclusions (and only such terms) have each been retained has been subject to post hoc analysis; the linearity of (87) plays an important part in such considerations.
Again, it can be confirmed that the above expressions remain valid at leading order under this scaling, notwithstanding the title of this subsection.
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Acknowledgments
This work was initiated with funding from Biotechnology and Biological Sciences Research Council and Engineering and Physical Sciences Research Council (BB/D008522/1), which we gratefully acknowledge.
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Appendices
Appendix 1: The limit \(a\rightarrow 0^+\) in (2)
The system (1), (2) contains two parameters, \(\mu \) and a; the bulk of this paper focusses on the dependence on the former, while here we briefly address the latter. The limit \(a\rightarrow +\infty \) is a regular one in which the ‘u / a’ term in f(u; a) is simply disregarded at leading order and the front is a pulled one (that the front is pulled when \(a\rightarrow +\infty \) for any \(\mu \) is a conjecture based on the results above; a stronger conjecture also suggested by our results is that \(a_T(\mu )\) is an increasing function of \(\mu \) with \(a_T(\infty ) = 1/2\), and \(a_T(\mu )\sim 1/\ln (1/\mu )\) as \(\mu \rightarrow 0^+\)). We are therefore concerned here with the limit \(a\rightarrow 0^+\) when pushed fronts are to be expected.
If we set \(\mu =\hat{\mu }/a\), \(c=\hat{c}/a\) and take the limit \(a\rightarrow 0\), (26) becomes
and it is clear a priori that the wave must then be pushed (the linearisation of f(u) being trivial); moreover, absorbing a by the above rescaling implies
and the preceding analyses of pushed fronts imply with minor modifications that
(cf. (22) and (74) as \(\alpha \rightarrow +\infty \)). It is noteworthy in (106) that \(\hat{c}_{{\text {min}}}\) scales with \(\hat{\mu }\) in the same fashion in both limits.
The scaling result (105) does not, however, apply when \(\mu \) is small with respect to a. For \(\mu =O(a)\), the distinguished limit in Sect. 5.3 applies, and a further regime with \(\mu \) exponentially small in a gives the transition from pushed to pulled fronts, as in Sect. 5.4. Thus, while for \(a>1/2\) fronts are pulled for any \(\mu \), for small a, pushed fronts occur except in a very small region of \(\mu \) parameter space.
Appendix 2: A linear differential–difference equation
The differential–difference equation
plays a central role in Sect. 5.4 and warrants brief separate discussion, not least because it constitutes a rare instance in which the appropriate treatment of a linearised problem in such a wavespeedselection analysis is not simply the Liouville–Green (or JWKB) approximation. The notation in this appendix differs from elsewhere.
Introducing the Laplace transform
(the poles of \(\hat{v}(\rho )\) have \({\mathbb {R}}(\rho )<0\)) gives
The poles of \(\hat{v}\) thus occur at \(\rho =\lambda \) with
(cf. (63)), so for \(\sigma <1/\mathrm{e}\) there are two (real) roots for \(\lambda \) that we denote here by \(\lambda _\) and \(\lambda _+\), with \(0<\lambda _<\lambda _+\), and it is easy to show that the complex roots all have real part larger than \(\lambda _+\).
Equation (107) requires initial data for \(0\le \xi \le 1\) and for our purposes in Sect. 5 it suffices to consider the case
for constant \(\nu \), and it then follows from (109) that
If \(\nu =\lambda \), with \(\lambda \) satisfying (110), we have
as is clear beforehand. More significantly for our purposes, the farfield behaviour
follows from (112) as a residue contribution; suppressing such a slowly decaying term is a central ingredient in the selection mechanism for a pushed front in §5.4.
Appendix 3: Pulled and pushed waves in a discrete diffusion equation with nonlinear coupling
In this appendix, we show that travellingwave speeds for (1), with nonlinearity given by (2), may be constructed explicitly in both the pushed and pulled regimes in the case for which constant coupling strength is replaced by the nonlinear function
in which \({\mu }\) is constant. The nonlinearity (115) converges to the constantcoupling case in the continuum (slowly varying) limit and is otherwise mathematically convenient, as we shall highlight below.
Travelling waves U(z), propagating at speed c obey
1.1 Stable–unstable connections
I Pulled waves Because
has \(\mu \sim \overline{\mu }\) in (115), the pulled wavespeed \(c^*(\mu )\) is again given by (4), (5).
II Pushed waves Wave propagation speeds determined by the whole nonlinear wavefront may be obtained by the ansatz (which motivated (115) in the first place):
We thereby obtain
and hence
with a pushed front occurring when \(\lambda ^\dag >\lambda ^*\); \(\lambda ^\dag (a,\mu )\), and \(c^\dag (a,\mu )\) can be determined explicitly in the form:
and the transition relationships \(\lambda ^\dag =\lambda ^*\), \(c^\dag = c^*\) imply that \(a_T(\mu )\) is given by
Equation (116) is evidently not of the class discussed in Appendix 3; moreover, setting
in (116) with \(c=c^*\) and W(z) slowly varying yields the dominant balance
so the farfield behaviour differs from (37), and the behaviour outlined in Appendix 3 might not be expected to pertain. Nevertheless, it is easy to see that \(\partial c^\dag (a_T,\mu )/\partial a=0\) also holds in this case.
Equation (121) implies
the former being consistent with the scaling argument embodied by (105) and the latter corresponding to the continuum limit. More importantly,
and it follows that
the latter is as expected from the continuum limit, but the former implies \(a_T\rightarrow +\infty \) as \(\mu \rightarrow 0^+\), i.e. the opposite behaviour from that in Sect. 5.4. Thus the analysis of (119) is counterproductive in terms of gaining insight into (26)—it does, however, reinforce the point that intuition into whether pushed or pulled behaviour is to be expected is hard to come by in the weakly coupled case. Figure 10 shows the curve (122) separating pushed and pulled waves, together with the weak and strongcoupling limits (127). The offset of the latter for \(\mu \ll 1\) illustrates further the implications of logarithmic terms in the associated asymptotic expansions (here through closedform expressions, in contrast to §5.4).
1.2 Stable–stable connections
For completeness, we exploit the analytic tractability of (115) to address this case also. For a connection between the stable states \(U=1,a\) (stable for \(a>0\)), representing the propagation of the state \(U=a\) overrunning \(U=1\) (and vice versa, denoted \(\tilde{U}\)), we set
where V(z) is defined in (118). In each case, the wavespeed and decay rate may be obtained as in Sect. 3.1. The resulting wavespeeds are
where \(c^+\) corresponds to the wave U(z) and \(c^\) to \(\tilde{U}\), and the decay rate \(\lambda (\mu ,a)\) is given in each case by
indeed the ostensibly distinct solution ansätze (128) in fact represent the same propagating front, moving in opposite directions.
Appendix 4: The generic pushed/pulled transition
Here we revisit briefly the analysis of Sect. 4 in a much more general setting. We consider the travellingwave problem
for a pseudodifferential operator with symbol P, whereby
and the analysis henceforth will for the most part depend on P only in the form of the function \(P(\lambda )\); for the discrete problem (1), \(P(\lambda )\) is
We again let
and assume P and f are such that for any given \(c>0\) a unique connection satisfying
exists. As \(z\rightarrow +\infty \) we will in general have (39), where \(\lambda \) is a root of
with smallest real part. The repeatedroot case has \(c^*\) and \(\lambda ^*\) determined by
in which case U satisfies (37). The transition value \(a=a_T\) is given by \(A(a_T)=0\) with, generically, \(B(a_T)>0\), and we again take the dependence of f upon a to be such that a pulled front arises for \(a<a_T\) and a pushed one for \(a>a_T\).
In the Liouville–Green approach (14), we have
on the envelope solution to the second of these (i.e. the Clairaut equation)
holds, so by (137) it follows that \(F=0\) on \(\eta =c^*\), \(\mathrm {d}F/\mathrm {d}\eta = \lambda ^*\), as is to be anticipated. Correspondingly, the expansionfan solution of the first of (138), parameterised by \(q=\partial \phi /\partial x\) (which is constant on rays) is
and determining where \(\phi =0\) gives a further derivation of the same result.
Again setting \(a=a_T+\delta \), \(\lambda \sim \lambda ^*\delta \varLambda \) and adopting the expansion (43) implies
from which we recover (44). Equations (136), (137) imply
a pushed front (whereby \(\delta <0\)) has \(C>0\), \(\varLambda >0\) (and hence \(P^{\prime \prime }(\lambda ^*)>0\)) so by (40)
and consistency with (44) demands
and hence
Finally,
the last term in which dominates the righthand side as \(z\rightarrow +\infty \) and implies
matching this to the corresponding term from (40) again implies (142), representing a useful consistency check.
Salient features arising from the above (providing one of the motivations for this more general analysis) include the following:

1.
The wavespeed \(c^*\) in the pulledfront case, given by (137), is independent of a; this is selfevident from the nature (134) of the linearisation, but is important to stress. Correspondingly, in the current limit, that the preexponential in (37) has no term quadratic in z has the consequence that (147) implies \(C\equiv 0\) for a pulled front.

2.
In the pulledfront case, the wavespeed is accordingly known a priori from (137); solving (131) for this wavespeed determines A(a) and B(a) in (37) and hence, through (144), (145), fully determines the pushedfront behaviour (wherein c has in general instead to be treated as an eigenvalue, being found as part of the solution) local to the transition.

3.
The result \(c^\dag  c^* = O( (a_Ta)^2 )\) as \(a\rightarrow a_T^\) seems to be generally applicable.
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King, J.R., O’Dea, R.D. Pushed and pulled fronts in a discrete reaction–diffusion equation. J Eng Math 102, 89–116 (2017). https://doi.org/10.1007/s1066501598293
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DOI: https://doi.org/10.1007/s1066501598293