Error with pdeCoefficient PDE toolbox

I am following this Matlab example to code my system of PDE (reequire PDE toolbox)

syms u1(x,y) u2(x,y)

pdeeq = [-laplacian(u1) + u2; -D*laplacian(u2) - pres];

symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

But if Try my Equation, I get an error and I cannot understand why. Any clue?

syms u1(t,x) u2(t,x)

pdeeq = [diff(u1,t) -laplacian(u1) + u2 == 0; diff(u2,t) -laplacian(u2) -u1 ==0];

symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 1-by-2.

Error in sym/privsubsasgn (line 1229)
L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/cat>catMany (line 42)
y = privsubsasgn(y,arrays{k},subs{:});

Error in sym/cat (line 25)
ySym = catMany(dim, args);

Error in sym/vertcat (line 19)
ySym = cat(1,args{:});

Error in untitled (line 7)
pdeeq = [diff(u1,t) -laplacian(u1) + u2 == 0; diff(u2,t) -laplacian(u2) -u1 ==0];

Answer 1

[Pratyush Roy]

Hi Edoardo,

As a workaround, you can try removing the equality operator and express the equations as a function of the variables. You can also express the equations in terms of the variables x and y instead of using t.

The script below might be helpful:

syms u1(x,y) u2(x,y)
pdeeq = [diff(u1,x)-laplacian(u1) + u2; diff(u2,y)-laplacian(u2)-u1*x];
symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

Hope this helps!

[Edoardo Saccenti]

thanks for your answer. Indeed this work but if i modify to

syms u1(x,t) u2(x,t) % or u1(t,x) u2(t,x) 
pdeeq = [diff(u1,t)-laplacian(u1) + u2; diff(u2,t)-laplacian(u2)-u1*x];
symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

will not work:

Index exceeds the number of array elements. Index must not exceed 1.

Error in sym/privsubsref (line 1201)
            R_tilde = builtin('subsref',L_tilde,struct('type','()','subs',{varargin}));

Error in sym/pdeCoefficients>extractC/extractblock3D (line 226)
        y = privsubsref(xy, 2);

Error in sym/pdeCoefficients>extractC/extractblock (line 174)
            [bl, remainder] = extractblock3D(eq, U, xy);

Error in sym/pdeCoefficients>extractC (line 283)
       [cblocks{n, p}, remainder] = extractblock(temp, privsubsref(u, p), xy);

Error in sym/pdeCoefficients (line 63)
[equations, C] = extractC(eq, fU);

Error in untitled2 (line 4)
symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

Index exceeds the number of array elements. Index must not exceed 1.

Error in sym/privsubsref (line 1201)
            R_tilde = builtin('subsref',L_tilde,struct('type','()','subs',{varargin}));

Error in sym/pdeCoefficients>extractC/extractblock3D (line 226)
        y = privsubsref(xy, 2);

Error in sym/pdeCoefficients>extractC/extractblock (line 174)
            [bl, remainder] = extractblock3D(eq, U, xy);

Error in sym/pdeCoefficients>extractC (line 283)
       [cblocks{n, p}, remainder] = extractblock(temp, privsubsref(u, p), xy);

Error in sym/pdeCoefficients (line 63)
[equations, C] = extractC(eq, fU);

Error in untitled2 (line 4)
symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)

Any clue?

Thanks again

Edo