## Specify Nonconstant PDE Coefficients

When solving PDEs with nonconstant coefficients, specify these coefficients by using function handles. This example shows how to write functions to represent nonconstant coefficients for PDE problems.

### Geometry and Mesh

Create a model.

model = createpde;

Include a unit square geometry in the model.

geometryFromEdges(model,@squareg);

Generate a mesh with a maximum edge length of 0.25. Plot the mesh.

```
generateMesh(model,"Hmax",0.25);
pdemesh(model)
```

### Function for Nonconstant Coefficient *f*

Write a function that returns the value $$f\left(x,y\right)={x}^{2}\mathrm{sin}\left(y\right)$$ for the nonconstant coefficient *f*. The function
must accept two input arguments, `location`

and
`state`

. The solvers automatically compute and populate the data in
the `location`

and `state`

structure arrays and pass
this data to your function. To visualize the `location`

data used by
the toolbox, add the scatter plot command to your function.

function fcoeff = fcoefffunc(location,state) fcoeff = location.x.^2.*sin(location.y); scatter(location.x,location.y,".","black"); hold on end

Specify the PDE coefficients using the function that you wrote for the
*f* coefficient.

specifyCoefficients(model,"m",0, ... "d",0, ... "c",1, ... "a",0, ... "f",@fcoefffunc);

Apply the Dirichlet boundary condition *u* = 0 for all edges of the
square.

applyBoundaryCondition(model,"dirichlet","Edge",1:4,"u",0);

Solve the equation and plot the solution.

```
results = solvepde(model);
figure
pdeplot(model,"XYData",results.NodalSolution)
```

### Anonymous Function for a PDE Coefficient

If the dependency of a coefficient on coordinates, time, or solution is simple, you
can use an anonymous function to represent the nonconstant PDE coefficient. Thus, you
can implement the dependency shown earlier in this example as the
`fcoefffunc`

function, as this anonymous function.

f = @(location,state)location.x.^2.*sin(location.y);

Specify the PDE coefficients.

specifyCoefficients(model,"m",0, ... "d",0, ... "c",1, ... "a",0, ... "f",f);

### Additional Arguments

If a function that represents a nonconstant PDE coefficient requires more arguments
than `location`

and `state`

, follow these
steps:

Write a function that takes the

`location`

and`state`

arguments and the additional arguments.Wrap that function with an anonymous function that takes only the

`location`

and`state`

arguments.

For example, define the coefficient *f* as $$f\left(x,y\right)=a{x}^{2}\mathrm{sin}\left(by\right)+c$$. First, write the function that takes the arguments
`a`

, `b`

, and `c`

in addition to
the `location`

and `state`

arguments.

function fcoeff = fcoefffunc_abc(location,state,a,b,c) fcoeff = a*location.x.^2.*sin(b*location.y) + c; end

Because functions defining nonconstant coefficients must have exactly two arguments,
wrap the `fcoefffunc_abc`

function with an anonymous function.

```
fcoefffunc_add_args = ...
@(location,state) fcoefffunc_abc(location,state,1,2,3);
```

Now you can use `fcoefffunc_add_args`

to specify the coefficient
*f*.

specifyCoefficients(model,"m",0, ... "d",0, ... "c",1, ... "a",0, ... "f",fcoefffunc_add_args);