# nonlinear coefficient in custom PDE.

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Prakhar Sharma on 14 Jun 2022
I am trying to solve a variation of 2D steady state heat conduction PDE using PDE toolbox Here , to simulate nonlinear behaviour.
Here is my code.
syms T(x,y)
syms k
pdeeq = diff(diff(T,x),x) + k*diff(diff(T,y),y)
symCoeffs = pdeCoefficients(pdeeq,T,'Symbolic',true)
symVars = [k]
symVals = [@(~,state) 0.01+1*state.u];
symCoeffs = subs(symCoeffs,symVars,symVals);
I get the following error, which does make sense
Error using sym/subs>normalize
Substitution expression X must be a symbolic, cell, or numeric array.
[X2,Y2,symX,symY] = normalize(X,Y); %#ok
Error in sym/subs (line 153)
Error in sym/subs>@(value)subs(value,X,Y) (line 64)
G = structfun(@(value) subs(value, X, Y), F, 'UniformOutput', false);
Error in sym/subs (line 64)
G = structfun(@(value) subs(value, X, Y), F, 'UniformOutput', false);
In case of standard heat transfer I can write this.
k = @(~,state) 0.01+1*state.u
thermalProperties(model,'ThermalConductivity', k);
How to do the same with my custom PDE?

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