# Solve Nonstiff ODEs

This page contains two examples of solving nonstiff ordinary differential equations using `ode45`

. MATLAB® has several solvers for nonstiff ODEs.

`ode45`

`ode23`

`ode78`

`ode89`

`ode113`

For most nonstiff problems, `ode45`

performs best. However, `ode23`

is recommended for problems that permit a slightly cruder error tolerance or in the presence of moderate stiffness. Likewise, `ode113`

can be more efficient than `ode45`

for problems with more stringent error tolerances or when the ODE function is computationally expensive to evaluate. `ode78`

and `ode89`

are high-order solvers that excel with long integrations where accuracy is crucial for stability.

If the nonstiff solvers take a long time to solve the problem or consistently fail the integration, then the problem might be *stiff*. See Solve Stiff ODEs for more information.

### Example: Nonstiff van der Pol Equation

The van der Pol equation is a second order ODE

where is a scalar parameter. Rewrite this equation as a system of first-order ODEs by making the substitution . The resulting system of first-order ODEs is

The system of ODEs must be coded into a function file that the ODE solver can use. The general functional signature of an ODE function is

dydt = odefun(t,y)

That is, the function must accept both `t`

and `y`

as inputs, even if it does not use `t`

for any computations.

The function file `vdp1.m`

codes the van der Pol equation using . The variables and are represented by `y(1)`

and `y(2)`

, and the two-element column vector `dydt`

contains the expressions for and .

function dydt = vdp1(t,y) %VDP1 Evaluate the van der Pol ODEs for mu = 1 % % See also ODE113, ODE23, ODE45. % Jacek Kierzenka and Lawrence F. Shampine % Copyright 1984-2014 The MathWorks, Inc. dydt = [y(2); (1-y(1)^2)*y(2)-y(1)];

Solve the ODE using the `ode45`

function on the time interval `[0 20]`

with initial values `[2 0]`

. The output is a column vector of time points `t`

and a solution array `y`

. Each row in `y`

corresponds to a time returned in the corresponding row of `t`

. The first column of `y`

corresponds to , and the second column to .

[t,y] = ode45(@vdp1,[0 20],[2; 0]);

Plot the solutions for and against `t`

.

plot(t,y(:,1),'-o',t,y(:,2),'-o') title('Solution of van der Pol Equation (\mu = 1) using ODE45'); xlabel('Time t'); ylabel('Solution y'); legend('y_1','y_2')

The `vdpode`

function solves the same problem, but it accepts a user-specified value for . The van der Pol equations become stiff as increases. For example, with the value you need to use a stiff solver such as `ode15s`

to solve the system.

### Example: Nonstiff Euler Equations

The Euler equations for a rigid body without external forces are a standard test problem for ODE solvers intended for nonstiff problems.

The equations are

The function file `rigidode`

defines and solves this first-order system of equations over the time interval `[0 12]`

, using the vector of initial conditions `[0; 1; 1]`

corresponding to the initial values of , , and . The local function `f(t,y)`

encodes the system of equations.

`rigidode`

calls `ode45`

with no output arguments, so the solver uses the default output function `odeplot`

to automatically plot the solution points after each step.

function rigidode %RIGIDODE Euler equations of a rigid body without external forces. % A standard test problem for non-stiff solvers proposed by Krogh. The % analytical solutions are Jacobian elliptic functions, accessible in % MATLAB. The interval here is about 1.5 periods; it is that for which % solutions are plotted on p. 243 of Shampine and Gordon. % % L. F. Shampine and M. K. Gordon, Computer Solution of Ordinary % Differential Equations, W.H. Freeman & Co., 1975. % % See also ODE45, ODE23, ODE113, FUNCTION_HANDLE. % Mark W. Reichelt and Lawrence F. Shampine, 3-23-94, 4-19-94 % Copyright 1984-2014 The MathWorks, Inc. tspan = [0 12]; y0 = [0; 1; 1]; % solve the problem using ODE45 figure; ode45(@f,tspan,y0); % -------------------------------------------------------------------------- function dydt = f(t,y) dydt = [ y(2)*y(3) -y(1)*y(3) -0.51*y(1)*y(2) ];

Solve the nonstiff Euler equations by calling the `rigidode`

function.

rigidode title('Solution of Rigid Body w/o External Forces using ODE45') legend('y_1','y_2','y_3','Location','Best')

## See Also

`ode45`

| `ode23`

| `ode78`

| `ode89`

| `ode113`