# How to specify initial conditions for a "Generic System" in the PDE App?

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Anna Ukolova on 14 Sep 2017
Edited: Svetlana Pease on 29 Sep 2017
Hello,
I'm using the PDE App (R2017a) and got a problem in defining the initial conditions. I defined a "Generic System" with a system of 2 hyperbolic PDEs.The problem is, that I've no idea how to define the initial condition within the "Solve Parameters" window for the two unknowns e.g. for u(t0). What I'm missing is an example about the necessary syntax used in the window.I should specify for u(t0) and u'(t0) functions of x and y for both unknowns u(1) and u(2).
May be you can give me an example for the correct syntax in the window?
The MATLAP Help says "The initial value can be a constant or a column vector of values on the nodes of the current mesh". How is the syntax to specify these "column vectors" to represent my functions depending on x and y??
Thanks a lot for your help!!
Anna

Alan Weiss on 14 Sep 2017
I believe that you should give u0 and ut0 as specified in the hyperbolic legacy solver.
Alan Weiss
MATLAB mathematical toolbox documentation
Anna Ukolova on 15 Sep 2017
Dear Alan,
thanks for you response! Unfortunately it doesn't work. First I've tried for u(t0) the expression
char('x.^2 + 5*cos(x.*y)','y')
and I got the error message
Then I tried as indicated in your link as an example for u'(t0) the expression
char('p(1,:).^2 + 5*cos(p(1,:).*p(2,:))','p(2,:)')
to get the nodal positions into play. But again I got an error message
So, I'm very helpless... I still does not understand, what is ment by " ...or a column vector of values on the nodes of the current mesh" and what is the MATLAB expression to achieve that!! Please help...
Thanks a lot, Anna
By the way: If I enter just constant numbers in the "char"-expressions, it's working...but I need the functions to apply the correct initial values at the nodes!

Svetlana Pease on 29 Sep 2017
Edited: Svetlana Pease on 29 Sep 2017
Dear Anna,
As a workaround, you can export the geometry from the app into the MATLAB workspace by selecting the Export Geometry Description option from the Draw menu. Then solve the problem using PDEModel.
Regards,
Svetlana Pease
Technical writer, MathWorks documentation group

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