# Problem with boundary condition for coupled pde using PDEPE solver

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Kartikey Sharma on 20 Feb 2021
Answered: Shashank Gupta on 23 Feb 2021
My boundary conditions are:
(left BC)
dCa/dx - vz*Ca + vz*Ca0 =0 at x=0
dCb/dx - vz*Cb = 0 at x=0
(Right BC)
dCa/dx = 0 at x=L
dCb/dx = 0 at x=L
f = [ dCa/dx ; dCb/dx ]
pl = [- vz*Ca + vz*Ca0 ; - vz*Cb ];
pr = [0 ; 0];
I am not able to define ql and qr here. I need to use 2*2 matrix for ql and qr but matlab donesn't allow me to do that and 1*1 matrix doesn't complete both of the BC. Any input is greatly appreacited.

Shashank Gupta on 23 Feb 2021
Hi Kartikey,
Check out this link, it has some example demonstrating boudary condition using matrix. Also if you want you can convert your matrix constraint to set of vector constraints then you might be able to solve them in typical fashion. Also what exact error did you get when you tried giving up the matrix constraint, Do let me know. I am assuming you are giving these boundary condition in the way shown in this link.
I hope this helps.
Cheers.