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# Dirac Delta function - as a solution to Modified Helmholtz equation

Posted 9 years ago
 So i am solving a differential equation which has a delta function. When i solve the differential equation, the solution comes up with a Heaviside Function for the particular solution of the DiracDelta. But in the book Arfken, we see that the solution for a modified Helmholtz equation with a delta function is Equation : p''[z] - k^2*p[z] = DiracDelta(z-z2) Solution in Mathematica : constansts + Heaviside Function Solution in Arfken : Exp[-kAbs[z-z2]]/2k How do i make sure Mathematica gives me the Arfken result instead of a Heaviside function????
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Posted 9 years ago
 I don't know of any systematic Mathematica procedure for solving this equation. You apparently can't put in f[Infinity] == 0 as a subsidiary condition. (Machines have not yet completely replaced humans!)
Posted 9 years ago
 CorrectionG(Z) = E^(-k|Z|)/(2k).
Posted 9 years ago
 The solution is a Green's function G{z,z2) = G{z-z2) = G{Z}, with Z = z-z2, which obeys the differential equation G''{Z)-k^2 G(Z) = -delta(Z). The minus sign is conventional. In:= DSolve[G''[Z] - k^2*G[Z] == -DiracDelta[Z], G[Z], Z] Out= {{G[Z] -> E^(k Z) C + E^(-k Z) C - ( E^(-k Z) (-1 + E^(2 k Z)) HeavisideTheta[Z])/(2 k)}} Now we need to determine C and C by the boundary conditions G[Infinity]=G[-Infinity]=0. From the first condition, as Z->+Infinity, E^(-k Z) becomes negligible, so the asymptotic solution approaches G(Z) = E^(k Z) C - E^(k Z)/{2k).This implies that C = 1/(2k). Setting C= 0, we find G(Z) = E^(-k Z)/(2k) for Z>0. The second boundary condition leads to E^(k Z)/(2k) for Z<0. Thus the final result is G(Z) = E^(k|Z|)/(2k).
Posted 9 years ago
 I agree with solution you have given, but how do i implement the same in Mathematica. Using the solution from this expression, I need to solve more Differential Equations.DSolve[{C''[z] - q^2*C[z] == -DiracDelta[z - z2], C[Infinity] == 0, C[-Infinity] == 0}, C[z], z]I tried giving the boundary conditions as the above mentioned, I ended getting an error message as shown below:DSolve::bvlim: For some branches of the general solution, unable to compute the limit at the given points. Some of the solutions may be lost. >>If you would be kind enough to give me the solution for this error.