# Input File

To access a user-defined function in AcuTrace, you must reference it in the input file.

```
USER_EQUATION( "energy" ) {
num_variables = 1
user_values = {5.0, 0.015}
user_string = {"test"}
type = evolve
}
```

This user equation command defines a user equation with one component governed by an evolution equation. The right hand side of the evolution equation is found by evaluating usrEner. The value of the user_function parameter is exactly the name of the function (routine) used in the C program written by you. The parameters user_values and user_strings are used to pass parameters to the user function. In this example, two floating-point numbers and one string are passed to the function. The order of these parameters is preserved, for example the first user value is 5.0 and the second is 0.015.

```
particle = massless
user_equations = {energy}
}
```

```
USER_EQUATION_INITIAL_CONDITION (energy) {
particle_seed = seeds
user_equation = energy
type = constant
constant_values = { 10.0 }
}
```

```
AUTO_SOLUTION_STRATEGY {
max_time = 0.0
max_segments = 10000
}
```

```
TIME_SEQUENCE {
...
staggers = {particle, energy}
...
}
STAGGER(energy) {
equation = user_equation
user_equation = energy
}
```

If the user equation is not so referenced, it simply retains its initial values throughout the analysis. See the AcuTrace Command Reference Manual for more details on the inputs for user-defined functions.