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differential equations - Using DSolve with a boundary condition at -Infinity


I would like to solve a simple 2nd-order ODE with one of the boundary conditions defined at −∞. The ODE I am looking to solve is:


w″


with the corresponding boundary conditions:


w(z=-\infty)=0, \; w'(z=0)=0+i\dfrac{\tau_{0}}{\mu}.


My attempt at a solution using DSolve is as follows:


DSolve[{-2 I \[Pi]^2 w[z] + (w^\[Prime]\[Prime])[z] == 0, 
w[-Infinity] == 0, w'[0] == 0 + I Subscript[\[Tau], 0]/\[Mu]}, w[z],z]


but I only get an empty set of curly brackets as an output. I checked the rest of my snipet of code without the w[-Infinity]==0 boundary condition, and that works as expected; therefore, I know that this is a problem with the boundary condition at z=-\infty. I am looking for methods with which I can solve simple ODE's with boundary conditions at infinity, and any help would be greatly appreciated.



Answer



This is the solution of your equation without the boundary conditions:


sol = DSolve[-2 I \[Pi]^2 w[z] + w''[z] == 0, w[z], z] // ExpToTrig //
ComplexExpand

(* {{w[z] ->
C[1] Cos[\[Pi] z] Cosh[\[Pi] z] + C[2] Cos[\[Pi] z] Cosh[\[Pi] z] +
C[1] Cos[\[Pi] z] Sinh[\[Pi] z] - C[2] Cos[\[Pi] z] Sinh[\[Pi] z] +

I (C[1] Cosh[\[Pi] z] Sin[\[Pi] z] - C[2] Cosh[\[Pi] z] Sin[\[Pi] z] +
C[1] Sin[\[Pi] z] Sinh[\[Pi] z] + C[2] Sin[\[Pi] z] Sinh[\[Pi] z])}} *)

Now let us take its limit at z->-Infinity:


Limit[w[z] /. sol, z -> -\[Infinity]]

(* {ComplexInfinity} *)

Let us now try this limit at C[1]=0 and C[2]=0:


Limit[w[z] /. sol /. {C[1] -> 0}, z -> -\[Infinity]]


(* ComplexInfinity *)

Limit[w[z] /. sol /. {C[2] -> 0}, z -> -\[Infinity]]

(* 0 *)

The latter gives us what we need, therefore, C[2]=0.


Let us now implement the second boundary condition:


    Solve[(D[(w[z] /. sol /. C[2] -> 0), z] /. z -> 0) == I*t/m, C[1]]


(* {{C[1] -> ((1/2 + I/2) t)/(m \[Pi])}} *)

Done. Have fun!


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