TransWikia.com

DSolve and coupled linear first order PDEs

Mathematica Asked on June 12, 2021

Does any one know a trick to make DSolve find solution to this coupled linear first order PDE system: (these are Cauchy-Riemann PDE equations, but with one of them having one of the dependent variables as well).

ClearAll[F1,F2,x,y];

ode1  = D[F1[x,y],y]-D[F2[x,y],x] == 0
ode2  = D[F1[x,y],x]+D[F2[x,y],y] == y  (*y here causes the problem*)

DSolve[{ode1,ode2},{F1[x,y],F2[x,y]},{x,y}]

Mathematica graphics

This can be solved in Maple:

restart;
#infolevel[pdsolve]:=3;
eq1:= diff(F1(x,y),y)-diff(F2(x,y),x) = 0;
eq2:= diff(F1(x,y),x)+diff(F2(x,y),y) = y;
pdsolve({eq1,eq2},{F1(x,y),F2(x,y)});

Solution it gives is

F1(x, y) = _F1(y-I*x)+_F2(y+I*x)
F2(x, y) = I*_F1(y-I*x)-I*_F2(y+I*x)+(1/2)*y^2+_C1

Screen shot:

Mathematica graphics

If the RHS of the second equation is not y but a constant or some other parameter, then Mathematica can now solve it:

ClearAll[F1,F2,x,y,m];
ode1  =  D[F1[x,y],y]-D[F2[x,y],x]  == 0
ode2  = D[F1[x,y],x]+D[F2[x,y],y]   == m
DSolve[{ode1,ode2},{F1[x,y],F2[x,y]},{x,y}]

Mathematica graphics

Is this a known limitation of DSolve or is there a trick or some other method to get the same solution as in Maple?

Using version 11.2 on windows 7.

One Answer

The following substitution eliminates the right side of ode2, and DSolve then can solve the resulting equations.

ode3 = Unevaluated[D[F1[x, y], y] - D[F2[x, y], x] == 0] /. F2[x, y] -> G2[x, y] + y^2/2
(* D[F1[x, y], y] - D[G2[x, y], x] == 0 *)

ode4 = Simplify[Unevaluated[D[F1[x, y], x] + D[F2[x, y], y] == y] /. 
    F2[x, y] -> G2[x, y] + y^2/2]
(* D[F1[x, y], x] + D[G2[x, y], y] == 0 *)

DSolve[{ode3, ode4}, {F1[x, y], G2[x, y]}, {x, y}] // Flatten
(* {F1[x, y] -> I C[1][I x + y] - I C[2][-I x + y], 
    G2[x, y] -> C[1][I x + y] + C[2][-I x + y]} *)

Correct answer by bbgodfrey on June 12, 2021

Add your own answers!

Ask a Question

Get help from others!

© 2024 TransWikia.com. All rights reserved. Sites we Love: PCI Database, UKBizDB, Menu Kuliner, Sharing RPP