# Documentation

## Initial Conditions

### What Are Initial Conditions?

Initial conditions has two meanings:

• For the parabolic and hyperbolic solvers, the initial condition u0 is the solution u at the initial time. You must specify the initial condition for these solvers. Pass the initial condition in the first argument or arguments.

u = parabolic(u0,...
or
u = hyperbolic(u0,ut0,...

For the hyperbolic solver, you must also specify ut0, which is the value of the derivative of u with respect to time at the initial time. ut0 has the same form as u0.

• For nonlinear elliptic problems, the initial condition u0 is a guess or approximation of the solution u at the initial iteration of the pdenonlin nonlinear solver. You pass u0 in the 'U0' name-value pair.

u = pdenonlin(b,p,e,t,c,a,f,'U0',u0)

If you do not specify initial conditions, pdenonlin uses the zero function for the initial iteration.

### Constant Initial Conditions

You can specify initial conditions as a constant by passing a scalar or string vector.

• For scalar problems or systems of equations, give a scalar as the initial condition. For example, set u0 to 5 for an initial condition of 5 in every component.

• For systems of N equations, give a string vector initial condition with N rows. For example, if there are N = 3 equations, you can give initial conditions u0 = char('3','-3','0').

### Initial Conditions in String Form

You can specify text expressions for the initial conditions. The initial conditions are functions of x and y alone, and, for 3-D problems, z. The text expressions represent vectors at nodal points, so use .* for multiplication, ./ for division, and .^ for exponentiation.

For example, if you have an initial condition

$u\left(x,y\right)=\frac{xy\mathrm{cos}\left(x\right)}{1+{x}^{2}+{y}^{2}},$

then you can use this expression for the initial condition.

'x.*y.*cos(x)./(1+x.^2+y.^2)'

For a system of N > 1 equations, use a text array with one row for each component, such as

char('x.^2 + 5*cos(x.*y)',...
'tanh(x.*y)./(1+z.^2)')

### Initial Conditions at Mesh Nodes

Pass u0 as a column vector of values at the mesh nodes. The nodes are either model.Mesh.Nodes, or the p data from initmesh or meshToPet. See Mesh Data.

 Tip   For reliability, the initial conditions and boundary conditions should be consistent.

The size of the column vector u0 depends on the number of equations, N, and on the number of nodes in the mesh, Np.

For scalar u, specify a column vector of length Np. The value of element k corresponds to the node p(k).

For a system of N equations, specify a column vector of N*Np elements. The first Np elements contain the values of component 1, where the value of element k corresponds to node p(k). The next Np points contain the values of component 2, etc. It can be convenient to first represent the initial conditions u0 as an Np-by-N matrix, where the first column contains entries for component 1, the second column contains entries for component 2, etc. The final representation of the initial conditions is u0(:).

For example, suppose you have a function myfun(x,y) that calculates the value of the initial condition u0(x,y) as a row vector of length N for a 2-D problem. Suppose that p is the usual mesh node data (see Mesh Data). Compute the initial conditions for all mesh nodes p.

% Assume N and p exist; N = 1 for a scalar problem
np = size(p,2); % Number of mesh points
u0 = zeros(np,N); % Allocate initial matrix
for k = 1:np
x = p(1,k);
y = p(2,k);
u0(k,:) = myfun(x,y); % Fill in row k
end
u0 = u0(:); % Convert to column form

Specify u0 as the initial condition.